Hello Paradraxor,
Sorry to hear about you're troubles with premades.
The best thing to do is take advantage of the Mute option in League of Legends. If you don't know how to mute someone, just press tab during game to bring up the scoreboard, on the right side you'll see a chat bubble icon for each person. Just click that icon and it will mute that individual person. If you want to mute everyone in a game, simply type "/Mute all" in the chat window.
If you feel as though these players are breaking the Summoner's Code, please report them after the game is completed so that they may face judgement.
Don't worry if they say they are going to report you as an empty report carries no value. If you yourself did not break the Summoner's Code, then there is nothing to fear.
The best choice is to always ignore abusive teammates since any communication with them is not aiding towards victory.
If there's anything else I can help you with, please don't hesitate to ask.
Have a great day! :)