Contact the company that processed your credit card transaction when you joined. (Most likely, Verotel)
Their direct email address is: firstname.lastname@example.org
Provide them your: Your Username for the site, and your subscription # (if you have it) and request that they cancel your recurring membership for AMG. They will then happily cancel any future rebills for you.
We do not process credit card transactions ourselves here. (duh lol)
If you purchased the 90-day non-recurring membership you do not need to do this, as your membership will not be rebilled.