Joel

User Spam Remover is a plugin for WordPress that automatically removes spam user registrations and other old, never-used user accounts. It also blocks the notification e-mail that WordPress normally sends to the administrator whenever a new user registers (annoying when that registration is spam!) and logs it instead.
The plugin adds a configuration panel so that all of these options can be turned on or off, and it logs and fully backs up all user accounts that it deletes, so that you can restore them if you need to.
Features:
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Automatically deletes user registration spam and other orphaned, never-used accounts.
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Very simple, enable and go! Doesn’t interfere with the normal user registration process in any way. So, it doesn’t add captchas or activation or anything else — you’re free to use it alongside a plugin that does, if you like.
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<li>
Blocks notification e-mail that WordPress normally sends to the administrator every time a new user registers (instead, logs this event).
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Fully configurable, with grace period for new accounts and optional username whitelist.
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Fully logs all actions and backs up all user accounts that it deletes so that you can seamlessly restore them if you ever need to.
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</ul>
Please see requirements and installation instructions below, or online <a href="https://wordpress.org/support/topic/official-installation-instructions-and-what-to-do-about-errors/" rel="ugc">in the WordPress support forum</a>.
For more information, please go to: <a href="https://lyncd.com/user-spam-remover/" rel="nofollow ugc">https://lyncd.com/user-spam-remover/</a>

User Spam Remover

Great way to clean a long running site of "junk" users.

Removed over 29k in minutes..

Effective

Does not work with many WordPress sites throwing a fatal error about mysqli.

Fatal PHP error about mysqli on activation

I had 31000 subscribers on a old (WP 4.4) website where I accidentially had the option "anyone can register" turned on.
This plugin did the job of deleting all those subscribers. I had to do it in batches as it "only" deletes 1000 subs at the time.

Thanks a lot to the developer as it has saved me a lot of hassle!!

It works!

Automatically removes spam user registrations and other old, unused user accounts. Blocks annoying e-mail to administrator after new registrations.

Tammy Hart

This plugin adds a page to the admin under Users that includes a form for users to send email invitations to their friends to register at your site. You can also add the form to the front end of your site with the <code>invite_friends()</code> template tag or the <code>[invite_friends]</code> shortcode.
IMPORTANT: This plugin will not allow invited friends to register unless you have registration turned on.
<h4>Features</h4>
<ul>
<li>Invite multiple friends at one time</li>
<li>Customize the email subject and message that is sent</li>
<li>Customize the URL to redirect a user to after they register</li>
<li>Store successful registered users in the inviter’s user meta data</li>
<li>Store the inviter in the new user’s meta data</li>
</ul>

Invite Friends to Register

Use this plugin to allow users to invite their friends to register on your site.

User Spam Remover vs Invite Friends to Register