If you need to vent, do it at your computer screen. Unbind the chat key so it is not easy to use. Learn to accept a probable loss when you see it, and don't jeopardize a potential win by harassing your teammates or the enemy team. If you get a feeder or afk, accept that you will probably lose, but use that time to practice playing in difficult situations. Practice strategies and see if there is anything you can improve or learn while playing the match.
Don't let another players actions cause you to get yourself banned. If they are trolling, don't rise to their bait, thats what they want, and doing so means they will win. If they are playing poorly and making bad decisions don't put more pressure on them by cussing them out. If you need to communicate a strategy keep it short. Something like, Teemo, we all need to group now. Or Teemo, don't push so hard. If they keep doing whatever they were doing before then ignore them and don't spam in chat. Use pings but do not spam them. If someone dies don't say a word, it won't help.
View self control as something that takes strength, maturity, and take pride in maintaining your cool. If you believe that you are a better player then your teammates, prove it with your gameplay and with your behaviour. Because honestly it isn't easy. I see a lot people claiming that they aren't pushovers and they have a right to defend themselves to any extent if they feel insulted or attacked. Or that they believe in being honest and calling it how it is. But will that really help at all? Be smarter then the average person. I have always thought that it takes way more self control to stay quiet even when you don't feel you should have too.
If you enjoy the game and any content you have acquired while playing it, then be willing to do anything that is required to make sure you don't lose it.