You could possibly do this without using a seperate hack using the "User promotion setting" under Usergroups in the admincp. start a user off in a "trial" usergroup with some access, and then auto-upgrade to "paid for" after 30 days, then if they choose not to pay they don`t have access, and then make sure you have the setting for a unique email address for each registration set, and ban the free email sites like hotmail, yahoo, etc.. and that might work, at least till someone comes up with a better idea