Then they get you for contempt of court and failure to pay, if you don't fight it you automatically plead guilty and they will hunt you down and jail you or garnish your wages or likely just outright take it out of your bank account, this is an easy to fight thing and people are already proving these fines as unconstitutional.

Frankly, if you refuse to pay and don't bother fighting it then you're an idiot who is jut giving them permission to take the money.