There are only two movies I have ever started to watch that I tapped out of because they were so fucking terrible. I'm sure there are plenty I've seen that sucked and are not worth another watch, but these two stand out as the only clear items on my "never again" list.
The first was Punch-Dunk Love. It was one of those films that's way up its own ass and trying hard to be artsy, but it was just awkward and extremely boring. Adam Sandler was surprisingly decent in a semi-serious role, but his OK performance was not enough to keep me from hating this movie enough to just turn it off about halfway through. This was back in the day of physical movie rentals being a thing, and holy crap was that a waste of a trip to the local video rental place and the few bucks it cost. >:[
The second was the new-ish The Great Gatsby with Leonardo DiCaprio playing Gatsby. He was a tightly wound bundle of awkward that was very fitting for the character. Everyone else though? Their acting ranged from boring to terrible (although to be fair to the actors it might have been more the crap script or crap directing that pushed them into sucking). The costumes and set design were fucking stellar, but they were heavily tarnished with the poor choice of using tons of flashy visual effects for no reason and using an anachronistically modernized soundtrack full of hip-hop instead of using period-appropriate jazz to fit the setting. The music was one of the things that really killed the music for me, because seriously, if you're going so far out of your way to make the visuals and the dialogue represent the 1920s, then why in the fuck would you diverge from that aesthetic with the soundtrack? Add to that the fact that Tobey Maguire's performance was truly awful and that he happened to be playing the main character, and that's a recipe for a movie turned off in disgust a little less than an hour in.