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).
</li>
<li>
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.

Functions:
<ul>
<li>Simple LogIn (one click)</li>
<li>Simple LogOut (one click)</li>
<li>Create user automatically</li>
<li>Update user’s profile with the informations that came from Identity Provider</li>
<li>Protect the user’s profile, that the user can’t modificate the informations that came from Identity Provider</li>
<li>Syncronize the LogIn and LogOut actions between the WordPress and the Identity Provider </li>
<li>If the (Apache) agent have a long update period you can choose what to do the plugin: go to login URL or LogIn.</li>
<li>SSL support</li>
<li>Rules: If a regular expression match to the $_SERVER variable’s element put the user to the group.</li>
<li>Plus plugin: Simple Group Manager – Manage the user’s groups and the group’s roles.</li>
<li>Admin page: Users / SSO Autentikáció</li>
</ul>
For more infromation please read this: http://kir-dev.sch.bme.hu/wp-opensso
If you don’t now how it’s work: https://redmine.kirdev.sch.bme.hu:8081/wiki/wordpress/OpenSSO_autentikacio (in Hungarian)

OpenSSO

Authenticate users using  Sun's Open Single Sign On service.

User Spam Remover vs OpenSSO