Attempts to collect debt not owed: Debt collection

I have disputed this item multiple times with the company and with the 3 major credit bureaus. I stated that I was unaware of this dept. I asked for validation and proof of how they keep verifying this. With who and how. I ask for a contract a wet signature, pay history or something and they just keep saying verified and they will not investigate it again without new information. How am I going to give information for something I have no knowledge of.