Not sure there's anything you can do at this point - you can sue, but that is costly. Unless you are talking about serious money it probably isn't worth it. You can set up an email drip, but they might block it.
I always structure deals so that payment is due on handoff, and at least 85% of the project has been paid out. If you budget for overages, for example by padding that 15%, you can either discount at the end or provide something extra for free. Tie payments to deliverables (40/40/15/15 or something like that), and attached a budget range to each. This way you can develop a relationship of trust with first time clients (who are also taking a risk), and by reducing the cost or providing extras to good clients you can get repeat business.
A friend of mine recently recommended an online service to get repaid. Addpaid.com, its supposed to be new and looks like an interesting approach to recovery. I havent needed it, but he used it and it worked for him. I hope for you as well