I will describe what he did, and just want to put more insight out there, his username is LazyPoop.
Basically, we all got into the champion select screen, and all started talking about what we wanted to play and everything went well. A few of us, me included, said we don't care what we play, and for everyone who does care to tell us now. We had a mid and a top, and an ADC, while I went jungle. Many times, we asked Lazy if he wanted to switch roles with anyone, as most people didn't care what they went. He gets up to pick, and auto locks Zilean with promote. After that, one guy said that he remembered him from last gaming, stating that he was a troll.
Lazy just sat there saying that he was trolling us, mad we gave him support, when we had clearly said many times that we would trade, and even offered before we locked in. So I had to dodge, but I wanted to report this guy for just being one of those problems with this game's community. I hope Riot has the champion select logs for this, because it was really stupid, and he apparently does this often.