Seo Dong-Chan I don't like any of his roles and I cannot stand to listen to him
Lee Minho. Lol overrated.
I don't hate him but I think Lee Min Ho is so overrated.Also, Hwang Jung Eum.I don't know why but I just don't like her and I skip watching shows which have her as the lead after watching just one of her shows
I just can't stand Ryu Soo-Young and I don't even have a particular reason. Well maybe it's because of his role in Endless Love but I never stopped giving bad actors a second chance *cough* *cough* Shin Se Kyung. I avoid all dramas and movies revolving around him. 
Actors with dirty scandal towards women like Lee Byung Hun, Park Yoo Chun, Park Shi Hoo, Kim Hyun Joong etc.

Well, hate is a strong word, so here are actors and actresses I dislike:
Ji Chang Wook
Park Hae Jin
Lee Min Ho
Sung Joon and his co
Jang Geun Suk
Nam Joo Hyuk and Lee Sung Kyung (their shippers made me disliked them)
Seo In Guk
Park Shin Hye
Suzy
Krystal
Uee
I don't really like Lee Jong Suk...
Song Joong Gi (he was annoying to me in Running man, but I like some of his characters)
Yamazaki Kento (overrated)
Park Yoo Chun - Never liked watching any of his dramas.
Lee Seung Gi - I don't hate him as such. But I've never felt excited watching him.
I don't hate them nor I dislike them (except for one) but they make me feel that I don't want to watch the drama if they're in ; 

park chanyeol.
lee soohyuk. 
lee seunggi. 
lee dongwook.
lee hyunwoo.
Lee Sung-Kyung - so annoying
Yoon Sang-Hyun - hate how he is talking
Park yoochun
Gong hyo jin
Shin se kyung
Uee
Hwang Jung eum 
Gu hye sun
Honestly, I slowly developed some anti feelings towards Lee Min Ho. I just feel like he is overrated. 
The first Korean drama that I ever watched was A Love to Kill from 2005 with Rain. I liked him a lot in that drama and thought I would enjoy his other works as well. Well damn, I was wrong. I actually dislike everything else from him.

Anything new coming from these two is something I'll just skip automatically.
I hate Lee Byung Hun the most.
Can't trust that person at all.
Probably same goes for Park Yoo Chun since scandal.

I dislike acting skills from Park Shin Hye and Hwang Jung Eum in any dramas.
Even I did manage to complete Kill Me Heal Me because of Jisung, Park Seo Joon (thank to Yo Na XD) and She was Pretty because of Siwon,
I kinda don't feel excited for Lee Min Ho and Jang Geun Suk.