vboptions > user banning options > banned email addresses and enter:
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Email address ban lists: You may ban any email addresses from registering and posting. Type in the complete email address (user@example.com), or use a partial email address (as in @example.com).
Note that partial email addresses are matched from the end of the address unless you enable 'Aggressive Email Banning' below. Therefore if you ban @example.com you will ban user@example.com, but if you ban @example that user will not be banned. If you enable 'Aggressive Email Banning', user@example.com would be banned by @example.
If the email address of a user attempting to register or change their email address matches any of the addresses you specify here will see a no-permission error. For example, if you have banned 'example.com' then a user attempting to use 'someone@example.com' will be rejected.
Put a space between each email address or email address fragment.