Collecting from deadbeat customers

the company (an LLC) probably doesn't own any property so there's really nothing I could put a lien on.

In that case for sure look into filing with a letter of attachment/lien on their company banking accounts. The letter they recieve will spell out whats going on and also inform them to be prepared to completely disclose any and all accounts to the court or they will also be subject to criminal prosecution. That letter from the court alone might scare the living hell out of them and get you a nice phone call with some real payment arrangements. Best of luck!