UsersWP Restrict User Signups Addon GPL Overview:
The Restrict User Signups add-on is your site gatekeeper. It will stop undesired users from signing up to your website.
Restrict User Signups Settings
You have the option to disable username restrictions and only restrict email domains.
- You may Disable Spaces in your Username.
- Restrict Full Match and Partial Match usernames. You can do that by entering a comma-separated list of undesired usernames.