With any such service, I budget 1 hour. If I can't get cancellation within that time-frame I have a standard form letter that gets sent out.
Once the letter is sent, after the current billing cycle ends any additional charges from said service are disputed (either on the card or on bank account) as fraud.
On three occasions, I've been asked to provide proof of fraud.
I've emailed a scan of the cancellation letter. The fraud has never been further disputed even from a gym in Chicago that (via their contract's language) demanded an in-person cancellation.
Life is too short and time is far too precious.