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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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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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.

wpgear

Ext Security. 
You may not know that your site has already been hacked. 
There are many ways to add a new user to the system, without your knowledge. 
This plugin will promptly notify you that a new user has registered on your site. 
Now the console has a widget that displays all new users.
All new users will be highlighted in red until Admin confirm each of them in User-Profile. 
With the active Option: “Deny Login if User is not confirmed”, you will sleep much more peacefully.
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<li>This plugin has already helped out many times when some of our sites were hacked. But we quickly found out about it. And we were able to fast stop the problem.</li>
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<h4>Features</h4>
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<li>Automatic scanning of the Users list on a schedule, and detect unauthorized addition to the DB.</li>
<li>Notification to Administrator by e-mail.</li>
<li>The “Users” Table has a sortable Column “Confirm” ON/OFF. Users who are not Verified are highlighted in red.</li>
<li>Option: “Allow to change Settings – only for Admin”. Default = ON</li>
<li>Option: “Deny Login if User is not confirmed”. Default = ON</li>
<li><a href="https://wordpress.org/plugins/adaptive-login-action" rel="ugc">Integration with “Adaptive Login Action”</a></li>
</ul>

New Users Monitor

Ext Security. Automatic scanning of the Users list, detect unauthorized addition. Informs immediately Admin by email. Informative Widget.

User Spam Remover vs New Users Monitor