In a sense, there isn't a single genre out there (to my knowledge, anyway) that I completely dislike and absolutely could not bear to indulge in, as there are always exceptions.

Anyway, the genre I dislike the most is sports. I'm turned off by just about anything where sports are the main focus, as I find them uninteresting. The exception? Sports that include some element of fantasy. From PangYa! to Mario Kart, if there is something fantastical about the sport, there's a chance I'll actually enjoy it. (Though, this mostly applies to video games.)

As for other disliked genres:

War: Really depends on accompanying genres and/or the setting, and if war is the main focus. I cannot stand modern warfare. I cannot stand futuristic warfare (as it's usually just modern warfare with more high tech stuff). I can totally indulge in media about wars between two kingdoms in a medieval setting, wars between superheroes, wars set in a fantasy world, etc.

Mecha: I do not like indulging in series that involves people fighting robots with other robots/people climbing into the robots and fighting others with them/what have you. However, I have seen some good mecha anime—Gurren Lagann and Code Geass come to mind—so I think it depends on how things are executed.

Documentaries: Depends on the subject matter and how it's presented. I fell asleep not more than twenty minutes into March of the Penguins, but found Super Size Me rather intriguing.