FDCPA has provisions for a letter with specific words to make them stop contacting you. They will likely ignore that, so consider if you would then sue for damages.

Most States have a 'declaratory judgment' law in which an issue is brought to court and decided. You could force them to prove a contract in a court. They will probably not show up. Or they could show up and you could get corrupt judge and lose.

https://www.law.cornell.edu/wex/fair_debt_collection_practic...