Admins can *carefully* do a mass-delete of suspicious users by:
- Visiting the Admin > People page (or YOURSITE/admin/people)
- Select all the rows with a spambot account
- In the "Operations" section, choose "Cancel user account" option click "Execute"
- On the pre-confirmation page, choose "Delete the account and its content" and click "Next"
- On the final confirmation page, make sure you are only deleting spambots and not real users or your admin accounts. Then click Confirm to delete the accounts.