Get their "new" address and "new" phone number from those whom they've defrauded.  Send a letter out to those who have been defrauded explaining the situation.  Provide those people with your business information including filing dates and registration numbers so that such things can be verifieds by them and it can lower the probability that some of them will come after you.

Meanwhile, do a reverse directory search on the phone number and address of the fraudsters.  Find out all that you can about them, even if it might cost a few dollars.  Track them down like the dogs that they are and gather information about the individuals behind it:  everything from BBB complaints, business filings, professional licenses, and homes owned.  Go to city, county, state, federal and private records. 

Also, file a criminal complaint against them.  The more that you can approach the authorities with, the better chance you'll have of stopping them from repeating this transgression with you or with anyone else here or in our field. 

These type of people deserve all the retribution that you can bring down upon their heads.  Have at it!