i was scammed

Not sure how this happened, but you would have to submit a support ticket.

Get rekt.

if u buy me a skin i can ban them for you :P jk obviously.. how did he scam u? so we are prepared

I told him i would buy him a mystery skin if he bought me one. i bought him it and he unfriended me

Submit a support ticket with the name and details of the event. Don't ask for your RP back - that's not going to happen. It will take some time before anything happens to him, while they investigate. Additionally, they won't inform you of any action they may take. However, skin-trade scamming is absolutely bannable, assuming that Riot is informed about it, and they will take action - assuming that their investigation is productive.