3
Preferences>User Info Settings
1. Enter emails that should not be used in user profile
xoops.org$|.cn$
The last bit stops registrations where the email address uses a .cn domain.
The works for me because there is no way that a legitimate user of my site would be from China. Might be a bit draconian for some, though.
2. I would also recommend setting "Allow users to change email address?" to "No". I had a problem where dubious accounts were being registered using dodgy domains and then setting their email address to fake addresses on otherwise bona-fide domains to hide their tracks.
