writ of execution

You need to send the other collection agency verification/validation letters in order to make them prove they have the right to collect on the debt. Make sure you send it either fax (and get a receipt) or certified mail so you have evidence to prove that you attempted to contact these people.
How did they get a writ of execution without taking you to court? Writs of execution can only be granted by court order as a result of a collection suit. With a writ of execution the sheriff can come and seize property. The only thing you can do to stop the writ of execution is to go into court (make sure you check for the affidavit of service in the case file) and appeal the judgment. I do recommend you get a lawyer to help you since appeals and getting judgments vacated is difficult, but if you win you can ask for your legal fees as part of your damages. Any decent collections attorney will help you with that since they'll want to get paid.