If you're in your series, I recommend calling their bluff.
If somebody announces feeding/trolling in champion select, especially when they didn't get their preferred role, they are trying to get somebody to dodge. And they are trying to get somebody to dodge because they don't want to lose.
Call their bluff, most of the time they'll dodge themselves. And if they don't you get to report them.
I completely understand if people don't want to do this in "regular" ranked games, since the LP penalty (and the lack of MMR penalty) is better than losing a game, but when you're already in your series, there's really nothing to lose.
Don't get me wrong, I really want Riot to implement a report function for champion select, and I hope this is not going to be seen as victim blaming. But the main reason why champ select trolling is so effective is because people give in so frequently.