I looked to see how to join and it indicated that ccbill was being used for membership billing. I found that
http://www.ccbill.com brought me to a page that included a cancel subscription button.
Clicking on that button, asks for some identifying information to allow them to search their records. If you still have the joining email, all the information you need is in that email. (I always keep joining emails so I can reference things like membership numbers or which email address I used to join.)
The interesting part is that if you joined for a month at $19.99, they will bill you every month following for $17.99. However, if you signed up for a year at $99.99, you get charged $2.99 every three months.
If you're on the monthly plan, I highly suggest you cancel and restart on the yearly plan. Things would need to be VERY slow for me to cancel since I only pay about $1 a month to maintain which is only $12 a year. It's a GREAT value. (unless they decided to change it.)