Collecting money from an attorney?

I own a business that has done services for an attorneys office. I've let several invoices for them slip because they were a new firm. They've now decided to not use our services but still owe us over $4,000. What is the best way to collect money from a law firm that believes they are above the law? I've already sent emails to them requesting a response but have received no communication in return. Any "real" advice would be helpful.