This is a little beyond the Amex forum but the contract between an individual and Experian describes how the contract works, how it is cancelled, etc.

I have no idea whether simply stopping paying cancels the contract or whether money continues to be owed until cancelled pursuant to the terms. Absent reading that, I wouldn't be giving my opinion on whether the OP continues to have a debt accrue. This isn't the lawn guy ......

The safest thing to do is always cancel according to the terms. And yes, these things make it hard to cancel and easy to auto-renew.

Get your free credit report one time per 12 months from each of the three major credit reporting agencies at: https://www.annualcreditreport.com/index.action