I'm think of making a mod where in admin accounts pending there would be another link / option to lock the user. But instead of adding the user to the members table and auto locking him it would put the email in a dead email table and on each registration that table would be checked and if the email is in there the user would get an error.
Should the table store anything in addition to the email address? Currently for the dbs I have
[CREATE]
DEAD_EMAILS
email#varchar(128)##
[END]