You will have agreed to have them debit your CC 28 days after signup, unless you have formally cancelled in writing. You cannot cancel until you have returned all the discs issued to you, and remember that these are posted out automatically as you return DVDs. So a couple of days before the trial expires, you need to empty your rental queue before returning the last disc, allow a day or two for it to reach them, and then cancel in writing and ask them to acknowledge that they have indeed received your last return.