Thanks for the suggestion, we will consider this for a future release.
The membership system was designed to allow your members to have access to your services as long as their payments continue to recur.
Your client has control to remove themselves from the membership at anytime (along with the recurring payments) by accessing their "my account" page. From their "my account" screen, they will see their recurring orders - and they will see their membership listed there. There is a "Cancel" button that will allow them to cancel their membership - which will prevent future charges.
They can also remove their credit card that is kept on file (or change the card number) in the top right corner of the "my account" screen.