Get a fingernail file, screw-driver, the lever on some toenail clippers, etc., and pop the enter key off of your keyboard when you sit down to play some league, then snap it back on when youre done.
Or, grow up a bit more, I find myself failing and being forced to mention we arnt winning by chasing them into their jg so stop, or say things like well Teemo ADC may not have been the best choice. These comments earning myself a report, because the emotionally stunted, self absorbed, "people" that play this game cant handle being told anything even if its obvious. After realizing this, its easy for me to keep my mouth shut again for a long while.
If you really want a long term solution, some others have mentioned this stuff here and in threads like it and I think you can find this stuff with a Google search, but unbind your chat key, edit the game files to disable chat, perhaps also; consider hypnosis, taste aversion, and electroshock to stop yourself from chatting.