Services rendered. They need to pay the bill. Trying to intimidate you by asking for your E&O is juvenile. Send them a final notice than nail them to the wall. Do everything you can, report it to Dunn and Bradstreet, report to the credit bureaus, send them to collections, file a small claims against them.
Report them to the BBB, File a complaint with the Bar Association, File a complaint with the Department of Insurance. File a complaint with the janitor's union if you can. The squeeky wheel gets the oil.
If ya really want some action, call their board member of the FLTA in their district.

George