About 96% of the historical dramas out there. It really has to call to me for me to watch it/or not drop it. And I hate dropping things...
Faith
Any drama with Park Shin Hye (2 dramas is enough)
Secret Garden
Moon Embracing The Sun
Any medical drama
Any thriller drama
Full House (Korean version)
Weightlifting Fairy Kim Bok
Any UEE drama
Any Military drama
i'll probably never watch heirs... and descendants of the sun xD
Descendants of the sun, thai full house
Boys Over Flowers. I just can't. I don't know if it's because it's too old now, but I just don't see me ever watching this. I had heard about how it was the gateway show for so many people to get into Korean drama, but I have a feeling if I had watched this one I would have run far away from it.
Any type of gender benders-.- I just loathe that genre was so much xD
Descendants of the Sun 
The Heirs
any  horror drama 
Heirs and BOF. They  don't  appeal to me at all.

also a drama I REGRET is HWARANG. Please don't torture yourself in watching it
High society. Hate the lead women especially whats-her-name-again, the one that's not uee
Historical dramas
"Guy dresses up as boy" dramas
Descendant s of the Sun (hate military stuff)
Historical dramas, never ever!
Like someone commented earlier, I really don't want to say I'll never watch something because that could change depending on what kind of mood I'm in. Also it depends on what's currently showing or how bored I am in my life. I have watched shows I thought I would never watch because I was so bored and decided to give them a shot and a few times have been so surprised. I've gone away thinking "why did I wait so long to watch that"?
I'm not sure about dramas, but one movie that I'm sure I'll never watch is the Crucible. I know what it is about and I am well aware that it would traumatize me. I read the reviews and the facts it is based on and I just can't. So if there is ever a drama that is really disturbing I can say with 100% certainty that I'm not going anywhere near it.
Pinocchio because I REALLY HATE PARK SHIN HYE .. although lee jong suk is one of my favorite actors ..I can't  handle watching  this park shin hye with him