Sorry you had a bad experience. When dealing with people like that, it's best to just mute them, report them and move on as they will get punished again by Riot (and yes I said again because they've most likely been punished before already). What's important to remember is to not engage the jerks in negative behavior yourself, as that just opens yourself up to being valid reported yourself by others in the game with you there. The in-game warning pop-ups only show when the system notices a spike in reports against you recently-that's it. It doesn't mean that you've been punished, it just means that you should make sure that your behavior is in check so that you don't get punished for it later on. Another helpful tip is if you have a jerk in your matchmade game, try waiting a little bit before queuing up again, as that increases the chances and odds that you'll get matched up with new people instead of with the jerk(s) again.
People try to make up excuses like that all the time and almost always wind up banned shortly afterwards as well. The following is a snippet from the Tribunal FAQ that is important to remember for the future (I know you said that you've only received warnings but regardless this is still important nonetheless):
That player in my game was far worse than me. Why am I banned???
I'll keep this one simple: those other players in your game that you thought were worse are probably also banned. Nearly 100% of the time, those players were banned even before you were. If someone is being toxic, just report them and move on.
We strongly believe that it is not OK to retaliate. If a player refuses to cooperate and says something negative, then players should ignore the player and report them after the game. Players can ignore or mute players by pressing the “Mute” button on the Scoreboard [when you hit TAB] in the game.
If a player retaliates and says something bad back, then the two of you have made a negative experience for potentially the other 8 players in the game. Rarely do teams start working together better when players begin fighting back—great games only happen when players cooperate and don’t make the issues even worse by fighting back and forth.
Basically as long as you're not being a jerk yourself then you have nothing to worry about, as the only ones who will be punished are the jerks and the false reporters rather than you.
