For me it's also Jang Geun Suk..
I don't know, he just annoys me and he looks like a girl with his long hair and he's way to slim. I don't know how he's considered to be one of the handsomest men in korea..
I can't finish a drama with him. I tried to watch "You're beautiful" and "Bel ami" but I can't watch him, I can't take him seriously somehow. It's kinda sad cuz I would love to watch "Love Rain" but I just can't, cuz he's the male lead :S
I really don't like Park Shin Hye...
There aren't any actors/actresses that I despise, but there are two that come to mind, who I find it very uncomfortable to watch:

Yoo In Na - There's something about her face that makes me slightly uncomfortable.  I get a strong impression of "dumb", "clueless", and "woman-child" when I look at her, which is strongly amplified by some of the roles she plays.  I'm also not a fan of her acting, either.  She's the one actress who I will be actively avoiding in the future.

Seo Woo - I think she's an amazing actress (I was highly impressed with her acting in "King's Daughter, Soo Baek Hyang"), but I find her face extremely disturbing.  For me, she's a classic example of cosmetic surgery gone horribly wrong.
I hate any actor/actress who ruins a drama for me because they can't act and hate it more when they are OVERRATED 
Actresses:
Jin Se Yeon, Soo Ae, Goo Hye Sun, Son Ye Jin, Han Ga In and Yoona

Actors: 
Lee Min Hoo, Lee Jong Suk, Kim Hyun Joong, Jang Geun Suk and Kim Jae Joong
There are many for me but boy does Lee Min Ho top the list.
Lee min ho
Park shin hye
shin se kyung
yoo in na
Haha I love this thread. I'm actually neutral towards LMH and PSH but I can understand that many people dislike them.

I also have some actors that I just can't enjoy for no rational reason:

Kim Hyun‑joong - I somehow get aggressice everytime I see his face.
Park Hae-Jin - His face always makes me feel unconfortable for some reason.
Jo In-sung - I don't know, he looks like a tortoise to me. I can't bring myself to like him.
Sung Joon - It's a stupid reason but I dislike his smile. But I saw him in one drama only, maybe I'll change my mind when I see him in another role.
Song Hye Kyo - Her acting has a ... idk, unappealing undertone that makes me dislike her.
Think i'm in the minority here but i can't stand watching Shin Min-Ah. I want to watch my venus but just can't lol. I'm not too sure why i dislike her either. Another is Suzy.

Actors ... ive had to think about this for a long time but one just popped out at me ... Lee Seung Gi. Like above, i really don't know why. And Mickey Yoochun

I was pained watching my girlfriend is a gumiho and if it wasn't for my boy crush No Min Woo, i wouldn't have seen it.
I feel that I'm more interested when I can read the reasons for your dislikes. So decided to provide reasons too.

Hwang Jung Eum. (Can't stand seeing her plastic face and acting. The most hated one, will never watch anything she is in)
Jang Na Ra  (Can't bear her face and voice)
Yoon Eun Hye. (Again, her plastic surgery'ed face feels uncombortable for me, as well as her acting)
Lee Bo Young. (Don't like her acting, facial and other expressions)
Song Hye Kyo. (Never touched with her acting, she has the kind of bitter arrogant taste. Whenever I see her I can't like her).
Ha Ji Won. (As she gets older, she overplays, don't like her acting more and more)
Kim Ha Neul. (Same as for Ha Ji Won)
Gu Hye Seon. (Something wrong with her eyesight, has no expressions)
Kim So Yeon. (I don't know, just very much not my type)
Shin Se Kyung, Kim So eun. (Just very pretty faces, but cant really act, both).
Han Ji Min. (Again tired with the unnatural face. Her acting is not bad though)
Han Ye Seul, kim tae hee. (Overplay)
Song Ji Hyo, Lee Ji Ah, Park Shi Yeon (Dislike at first sigh)
Park Min Young. (Under and overacts, fake)

Bae Yong Jun. (The plastic face feels uncombortable for me).
Jung Yong Hwa. (Uncomfortable face on the screen. I saw him live and there he looks so different, much better)
Jang Geun Suk. (The plastic face! Too artificial...)
Kim Hyun‑joong. (Same as for Jang Geun Suk)
Park Yoo Chun. (Annoying acting)
Park Hae-Jin. Hmm. (I like his roles, but his face is too uncomfortable, yes)
Lee Jong Suk. (Just already really tired with unnatural surgery'ed faces + don't like his acting, it looks rather like posing and modeling)
Joo Won. (Sick of his characters and kinda dumb expressions)

There must be more, but enough for now.

Kim Hyun Joong -- I hated him the minute his scandal broke. He has a very scary face ... he looks like a guy with a lot of rage in him.

Kim Ha Neul -- should retire from acting immediately, just like her compatriots Ha Ji Won & PSH. This woman has no range and she plays all her characters with the same one note & an undercurrent of meanspiritedness. It's all in the eyes (same with HJW). The mirror to the soul. A lot of resentment in these two (towards what I dare not speculate ... aloud) and it comes out in their portrayals, no matter the character they play.

Lee Bo Young -- actually she can do a fair job if given the right role, but I get the same impression of her as with KHN above. Moreover, unlike HJW and KHN, I don't dislike LBY, I just don't like her either.

Hwang Jung Eum -- Her face, her voice ... can't do it. Just no

Pretty much anyone who is TOO overrated I'll start to dislike because I think it's unfair that more talented actors will never get the recognition they deserve.

Having said that, I actually don't mind Lee Min Ho. It's not because I think he's hot. I am actually not a fan, and while he's clearly not ugly, he's not the world's most gorgeous man either. Not in my eyes anyway.

I am solely judging him based on his acting and to me he's definitely overrated, especially given all the dumb dramas he's played in (no disrespect to the fans) and certainly definitely because of them too.

After seeing him in Gangnam 1970, however, I think he has the potential to develop into a decent actor if he chooses his roles more wisely. It wasn't that this was a great role overall, but the movie showed that he has the capacity to portray a wide range of feelings & emotions. He just needs to start doing more serious, grown up roles (after all, he's a big man now, not a kid anymore!) and stop with all the teeny boppy sh--. What the heck is he doing accepting roles like Kim Tan in Heirs at his age??? Big man pushing 30 ... come on now, son!

Nanotao.. you mentioned all the good K-actresses I know.. i'm really curious about the ones you like =/

For me, THE ONLY ACTOR I cant really stand is Jang Geun Suk.. He always plays the same arrogant-flower-boy character and that annoys me because I don't find him handsome at all to start with! Plus, he's a very bad actor.
There are other actors I don't like (Lee Min Ho, Lee Jong Suk, Park Yoo Chun, SoJiSub... and others i don't recall now) but not as much as JGS.

For actresses I don't hate any, I just don't like Gong Hyo Jin's style (all of it, from her haircut to her clothes & shoes), and there is something I don't like about Ha Ji Won but I can't describe it, her aura, maybe :P .. but I always watch their dramas if I liked the plot.
Ah, there is one i would never watch any of her dramas: Gu Hye Seon.
Lee jin wok
Not despise but I don't like him for no reason. lol
Lee Min Jung,
Moon Geun Young,
Kong Hyo Jin,
Shin Se Kyung,
Yoo In Na,
Yoona (i can accept her as singer, but not as an actress, idk why)

I really dont know why, it's not like I hate them, also not liking them, it just haihh, I really cant describe XD
lee bo young
jang nara
yoona