Hi @gail
Looks like your site have been hit by spammers.
- First you need to disable your mail (ie: edit your config.inc.php) to avoid more mails.
- Second, enable catpcha and honeypot plugins to avoid more fake users.
- Finally, you need to clean your user list going to your DB and checking for spam user’s patterns (or doing it manually). This post could help you with this last task.
Once you finish, you will be able to renable the mail and go back to normal.
Cheers,
m.