I don't really hate hate any drama actors but for some reason Hyun Bin gives me bad vibes haha.
Jang Geun Suk
Jang Na Ra
Jung Yonghwa
Moon Geun Young
Lee Minho
Sung Joon
Park Shin Hye
Kim Tae Hee

I don't despise them but I don't really like them as actors and I think they are overrated...
Park Shin-hye - I don't know. I just have these random dislike for her, and I can't seem to like anything that she've been starred except for You're Beautiful. I love her for her beauty, but acting-wise, I don't think she'll ever satisfy me with her acting.

Han Ye-seul - I first saw her on Myung-wol The Spy and wasn't really impressed w/ her back then. And then she began to be active on dramaland since Birth of A Beauty, but I just don't like her role there which made me to put Birth of A Beauty onhold. And then I saw her again in Madame Antoine, but there's something in her facial expression that made me dislike her.

Lee Min-jung & Jin Se-yeon - Idk why but they just gave off vibes that I don't like. 

Lee Jong-suk - I don't particularly despise him, but I find his acting a lil bit lacking. I love him on School 2013 but his latest dramas just don't pick my interest even though he's loved more bcs of those dramas.

So the main point is whenever I dislike a certain actor/actress, it just bcs they often give off vibes that I don't like. It was quite random, but I strangely felt like that whenever I see these people above xD
Jang Won Yeong - something about him just seems shifty. Slimy.  I've seen him in two shows: Let's Eat & Queen In Hyun's Man, 
Take a look at my blacklist, http://mydramalist.com/list/X3bgn0B4

I don't despise them though.I just I avoid watching them for some reasons, like their voice, the way they talk, their obvious plastic surgery etc. although there are also dramas that they're okay. I still consider when I'll watch them or not, but mostly No. xD
My top 3 actresses and actors to avoid...

1. Jin Se Yeon - I saw her in Bridal Mask and she was just horrible... the same face during whole show, whenever I see her in the cast list I'm not going to watch this drama
2. Gong Hyo Jin - I tried 3 dramas with her and it was always a failure
3. Joo Sang Wook - this guy really can't act (in my opinion), first I saw him in Good Doctor and his smile was so fake, now I'm watching Special Affairs Team Ten and it's even worse

Others whom I don't really like but I'm willing to give them another chance:
1. Park Yoo Chun
2. Song Seung Hun
3. Kim Tae Hee
4. No Min Woo (I just can't forgive him leaving his rock band The Trax, he's such a talented musician, why acting?)
5. Soo Ae (there's something about her face...)
6. Jung Woo Sung
7. Jo Jung Suk
shin se kyung.. totally agree! i can't stand her acting.. very unnatural!
While I wouldn't say despise, I'd say I strongly dislike Park Shin Hye enough to not watch anything where she's a main role. Something about her acting comes off as really flat/sameysamey every time. I'd probably only watch something with her in it if My love for the other actors was strong enough to put myself through it. Otherwise, I can think of characters, but no actors or actresses that I absolutely can't stand...
OMG same same! I don't like Park Shin Hye and Lee Minho either!! I don't get why soo much people like Lee Minho. I also don't like Shin Se Kyung.
http://mydramalist.com/people/1104-song-ok-sook

She over acts in everything.  I don't know how she gets work.  I can't stand her
Park Shin Hye - She has a nice-ish face, but her acting in Pinocchio was aids.
Jeon Ji Hyeon, even though You who came from the stars is one of my favorites, I can't stand her >_<
Jin Se Yeon is the second I can't stand. Something to do with how she acts everytime <_<
Kim Tae Hee, Park Shin Hye and Kim Hyun Joong.
Park Shin Hye 
Lee Min Ho
IU
The people i really can't stand;

Actresses:

Shin Se Kyung
Song Hye Kyo
Yoon Eun Hye
Han Hyo Joo
Nam Ji Hyun 
Moon Geun Young
Kim Tae Hee
Park Shin Hye
Goo Hye Sun
Yoona
Ha Ji Won
Hwang Jung Eum
Suzy
Wang Ji Hye
Lee Bo Young
Uee
Lee Da Hae
Seo Ye Ji
Kim Ha Neul
Han Ye Seul

Actors:
 
Lee Jong Suk
Kim Hyun Joong
Lee Seung Gi
Lee Hong Ki
Kim Jeong Hoon
Kim Soo Hyun
Yoon Sang Hyun
Lee Jin Wook
Jang Geun Suk
Yoo Seung Ho
Choi Min Ho 
Joo Sang Wook 

The reason i can't stand these people is mainly because they have zero acting skills or they have a plastic face.