The gym won't let you leave. Write the letter that changes that.

Your minimum term ended, you have asked to cancel more than once, and the gym keeps helping itself to your account anyway. This drafts a cancellation and refund demand citing the Consumer Rights Act 2015 on unfair terms, demands every post-term charge back within 14 days, and states you will raise a chargeback and report them to Trading Standards. For anyone the gym refuses to let leave.

Draft a cancellation and refund demand to [gym name]. My minimum contract term ended on [end date] and I have requested cancellation [number of times]. They have taken [number] unauthorised payments of [amount] since then. Cite the Consumer Rights Act 2015 on unfair contract terms, demand a refund of all post-term charges within 14 days, and state I will raise a chargeback with my bank and report to Trading Standards if unresolved.