How can the Nevada State Athletic Commission enforce the fines they sometimes give to athletes? I.e., if an athlete refuses to pay their fine (example), what can the Nevada State Athletic Commission do, aside from revoking their license to compete in Nevada?
Well, from the reference you link:
The commission, which is an agency of the Nevada Department of Business & Industry ...
As part of the government it can throw your ass in jail if you don't pay the fine. They would do this by following the due process of law. Basically the procedure would be to request an arrest warrant from a court, arrest you and bring you before the court.