Tough it up, I also got banned for saying KYS in 1/11/2017 and I played 200 games to get Platinum, I was gold in the other 2 queues.
You will feel like shit because you won't get rewards, and you won't be able to play for 2 week on your main, but after 2-3 days, you will get over it.
Just accept the fact that some people in this world think that's offensive. For example, I think the N word is overreacted, kinda silly and meaningless, so me and my friend sometimes use it for comedic purposes, but not with other people around, we respect the sensitive people and won't say it.
Me, you and everyone that was banned 15 days deserves the ban, think about it, you are getting treated like shit because you treated others like shit.
The new runes are out, just go on a smurf (or create another account) and force yourself not to flame, you will see that it's much better to not be negative.