one way you can prevent people just registering themselves is to set the RegistrationRequiresApprovalP flag to 1. This will have the effect that, although people will be able to complete the registration forms, their users will not be activated without an administrator specifically doing that.
You set this flag by going to /admin/sitemap and clicking "set parameters" next to "Main Site" (near the bottom). Then click the "user-login" section link and look for the RegistrationRequiresApprovalP parameter.
If you really want, you can cosmetically remove the ability for someone to get to the registration pages by hacking the login pages in the acs-subsite package (off the top of my head i think everything you would need to change would be in packages/www/register)
