You can view all of your customers who have active memberships with you from your AllProWebTools Console. Just choose Reports -> Sales -> Memberships from the main menu bar on the left side of the screen.
On this page, you will see a list of all active customers with memberships. When you find the customer that you would like to cancel membership for, you can click the green button that says "active" to deactivate the membership (this will allow you to restart the membership at a later date if you like) or you can simply click the red "delete" button to remove the recurring subscription completely.
Either of these methods will stop any future charges from your Authorize.net account.