Send them a certified letter, wait a week and then call. Record the call, tell them it's recorded and mention the letter. Tell them that your account is cancelled and you're no longer paying. Then don't pay, and or charge back. If they threaten to send you to collections, let them. If they actually do, just tell the collection agency the debt is invalid and send them proof.

Just don't pay. I have had to do this a handful times in my life, and it has never hurt my credit score or went anywhere, never even needed the evidence I collected. Collections will just give up after a while, and if it somehow ever does become an issue you have all the proof you need to show its not valid debt. But, it likely won't actually go anywhere.

I did have one really annoying collection agency one time that really didn't seem to want to let it go. So I started generating invoices for my "research time" and send it to them. When they would call I would start the call by saying "by continuing this conversation, you agree to pay for research fees". Not sure if that did anything or they just coincidentally gave up… I was kind of hoping their AP department would just blindly pay the invoices for my time haha.