If another summoner breaks the summoners code, there is no problem with muting
By muting one team mate, you aren't refusing to communicate with the rest of the team, you are simply refusing to accept their harassment.
I have only had to use the mute button once or twice, as I try and save for the last resort. However, it has changed games for me and I advise players to use it whenever they start to feel the rage.
Another good start to avoiding rage is to play with friends! If you don't have "real life" friends who play LoL, find a group online. I recommend adding people after you lose a game, not after you win. After you win a game, every one will be happy. After you lose though, if one player continues to be graceful, they are probably truly a good person to play with.
Good advice TheFlyingSpoon