Geez, I am so sorry this is happening to you and want to know when this slimy company will finally be shut down!
First off, I would send them a Cease and Desist letter to their address in Nevada. They'll ignore it, but your ducks will be in a row. Next, file complaints against them with the BBB, the FTC and the Texas State Attorney General.
So do the Cease and Desist and Validation letters (where you ask for proof from them that you owe this debt) and then file your complaints, mentioning to these agencies that you have done this.