This is why every landlord should make sure and get/verify social security numbers for everyone on the lease. If you have a civil judgement you can garnish their tax returns. And I'm going to guess that quite a few of the deadbeats get a tax return every year in the form of refundable tax credits. Doesn't help in the short term, but may in the long.
I've known some such dirt bags, and they don't bother to file a return because they know they aren't getting any money. But... everything helps. In 20 years if they forget and do file a return, bang, money is headed your way.