You can cancel the recurring charges at Epoch or CCBill depending on what payment processor comes up when enrolling. Over at those sites, you would go to a secure area within those sites. You can enter your email address and another form of identification - you need two - (i.e., your membership number, your subscription number, possibly your credit card number (some payment processors like CCBill and Epoch will require the whole number; while at others like Segpay, Verotel, AXS, and Gemma Entertainment (the Evil Angel sites and Fame Digital, 21Sextury) may just ask you for the last four numbers. If this was Brazzers for example, Pro-Biller would ask for the first six and last four numbers of the credit card. Of course, as Uchual says better to do the cancellation before the next billing cycle.
Whatever you decide, do not "chargeback" the claim - act like you didn't authorize it. If you do that -- then the payment processor can bar you later if you do decide to change your mind and try to re-enroll again. You may be blocked depending on what the webmaster decides. Remember, the website has to incur the cost of your membership especially on charged backed accounts. They won't like it.