That would be fairly easy to do I think.....
Sort of add a check in whatever scripts that you don't want them to have access to, if their post count is > 100 and they aren't a member of the "subscribed" group.... then block them.
JustDave's double identities mod uses a cookie to check for multiple usernames using the same computer... that would help you catch duplicates.
They could sneak by you if they delete their cookies though. So it's not perfect.
