Im mad someone stole my uncreative username. Should they give it back?

I'd message them like "i'm sorry but you'll have to change your username. I called dibs and you know the rules, thanks xxx"

If they didn't know you at any other site and used it here before you did, it is theirs. But, if they did know you at other sites then are using it here, then you have a right to take it back. If that is the case, notify the admin, with proof, thinking you are being impostered.