So i got banned for 2 weeks. Because i "flamed" my premade friend. we were joking and he knows that we were even in TS. anyway riot decided to ban me becuase i called him a %%% 3 times. i actually never flame when someone plays bad. 
Would it change anything if i write them?
The word you mentioned there, while used in some friend groups as a lighter insult, is considered a hate speech by the bulk of society. If it is a word you are comfortable using among your friend group, that is fine, as long as you only use it among that friend group. That means if you are in a game with other players who are not your friends, you should not use that word in team chat.
Riot has a zero-tolerance policy for exposing other players to that sort of hateful language. It is almost certainly not going to change, even if you write to them to explain the situation. The simple fact is that you exposed a player to hate speech against that players will, and that is considered unacceptable behavior.
Saying these things to a friend doesn't make you invulnerable to suspension nor any restrictions. You shouldn't be saying these things at all. Regardless if you're friends with the individual or not. You're not permitted to flame in general using Riot Service as you agreed that you wouldn't in section 5. Code of Conduct at section 5.1. The League of Legends chat is meant for friendly conversations, strategies, and friendly/positive constructive feedbacks.
Remember, after a 14-day ban, you get one more chance before permaban to change your actions in these games. If you need to say anything to a friend, a joke or anything that is mildly too offensive for League Chat, say it in Discord, teamspeak, ect.
