In most cases you can. If you've taken reasonable steps to cancel in good faith and the merchant is uncooperative then canceling the payment is the logical next step.

They can still send your account to collections or ding your credit record, but you can still refuse to pay; ultimately the only way they legally force you to pay is by going through the courts system but I expect that if you have documentation of your (unsuccessful) efforts to cancel the case should be in your favor (which is why this will never reach the courts in the first place).