you could unauthorize their user, and also set the requiresUniqueEmail setting to true in web.config to stop them creating a new account with the same email (note: this affects all users)

In addition to requiring a unique email address...
In addition to the default Security Roles, you can create an additional Security Role named Current Member, or even create Economy, VIP or Gold Class to have 3 different levels of membership.
The pages can be set so that only those security roles can see the pages, or if you want to tease new users to purchase you can set the page display to all, then have one text/html block  with "all" access saying "click here to join via Paypal",
and the real content on the page can be limited to Economy, VIP or Gold Class etc. by editing the viewing permissions of that module.

If troublemakers or confused people ("id10T") continually login and don't get a greater level of access, they will soon stop.  You just need to clean out the non-members more frequently.

BarryZ