post a buyer beware thread about him using his username in the title.
the beware thread must be completely filled out or it will be deleted.
proof is the key. It will take some work to complete the beware thread correctly.
also, use the report button on your post here to gain more moderator attention and list his usernames in the report. We will research him and if he has a negative history, we may ban him directly.
banning him will not help you get your winnings but it will prevent others from having problems with him.