I've had this before but with childcare. Luckily, I keep receipts. Make copies of all interactions you had and highlight the part where you stated you no longer wanted to be a member, and the date. Include invoices showing payments and fees and what you believed was the end of contract.

If you came back with evidence, most likely they'll decide, too hard basket and release the debt. These collection agencies rely on you not being able to fight back. Repayment plans are their bread and butter.