In AllProWebTools, the membership will remain valid for the entire length of time the membership is valid for. It will be automatically cancelled, and access to webpages revoked, whenever the next membership payment is not paid.
However, you can cancel a membership at any time manually under Reports->Sales->Memberships. Once on this report, you can sort the list by "Name," "Company" or "Membership" by clicking on the table headers, and find the customer/membership you need to deactivate. Then click on the "Status" switch to deactivate the membership!
Please let me know if you have any more questions!