Yep. Premades are often quite cancerous. What's more, they'll usually not only gang up on you with abuse and harassment (not to mention reports) but attempt to get the enemy team to report you as well. And sometimes it will work, since "well if four of them are complaining about that guy, he MUST have been toxic".
There's not much you can do about it though, alas. If I think I'm in a game with a 4 man, I just stay quiet the entire time. No matter what. Even if they talk to you. Oh, listen if they're asking you to do something that makes sense, like group up or come help with Dragon or something... but don't talk to them, don't offer advice, and don't present yourself as someone to rage at. If they're playing goofy and dumb, just accept that the game might be a loss and have fun yourself. It sucks, but think of it this way. If you were on a team of 5 randoms and all but one guy wanted to rush drag... so that one guy ignored everyone's wishes, instead pushed mid alone, got caught and died, and let them rush down mid 4v5 and take an inhib or win.... you'd blame that guy, not the team. TRY to work with the premade. Just... don't chat with them... and don't expect them to deviate from whatever dumb idea they have in mind for a silly thing like YOU enjoying the game. Try to have fun, let them act dumb if they want, and accept that there are other games. If they're toxic despite this, then report them and hope for the best. There will be other games to play. No sense getting hung up on that one.