1.) Pay out of your pocket, ask a lawyer to send ex-employer an letter of intended action plan if not pay up + cost of the letter.
2.) If finally no response, and the amount less than $5000(limit set by local state), you can take ex-employer to small claims court, or hire a private lawyers to take ex-employer to court.
3.) Report the incident to local state/city like labor department, get public prosecutor to write them a warning letter. This often free of charge. Will scare the shit of the ex-employer. I am for sure they will mention jail time and heavy fines in the letter if not pay up.