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.

Jan Štětina

Warning: This plugin is currently broken (at least for some sites) and I have no time to maintain it. I plan to solve all the issues from the support forum but I have no idea when that might happen (perhaps summer 2014). I am sorry and thank you for the understanding.
Provides a system for registration requests and their processing in multisite.
Blog administrator can place a shortcode <code>[murm-form]</code> on their blog, which will display a registration form. Visitors can then use this form to request registration. Such a request must be first moderated by the blog admin. If it is accepted, it’s passed on to the blog superadmin, who also has to approve it in order to pass. After approval the plugin creates given user accounts with subscriber rigths on the blog from which it was requested and sends login information to the user.
Per-blog settings
<ul>
<li>can be deactivated</li>
<li>notification via nag or e-mail on new requests</li>
<li>antispam (needs Akismet API key)</li>
<li>requests on the blog admin level can be automatically approved</li>
</ul>
Per-site settings
<ul>
<li>optional e-mail notifications</li>
<li>custom e-mail templates</li>
<li>blog administrators can be allowed to delete a request without notifying it’s author</li>
<li>requests on the network admin level can be automatically approved (this means that only blog administrators decide about their requests, if no error occures)</li>
</ul>
This plugin was developed for a small blog server (cca 30 blogs), so that superadmin would have control of newly created accounts (because some people were confused and got themselves unneccessary multiple user accounts).
Makes use of the <a href="https://wordpress.org/extend/plugins/wordpress-logging-service/" rel="ugc">WordPress Logging Service</a>.
In future there will be more options to modify default plugin behaviour.
<h3>Wishlist</h3>
Below are listed requests I know about and I’m going to process… eventually. Please be VERY patient.
<ul>
<li>“I have a request of asking for password on registration form. That way the user can pick out a password and have it automatically populated in the site user data.”</li>
<li>sidebar registration widget</li>
<li>attach custom admin message to user on request approval/denial</li>
<li>correct i18n, custom blog admin messages</li>
<li>custom shortcode css (error/ok) + information</li>
<li>save all settings as a single wp option</li>
<li>combine with pure-murm?</li>
<li>“I would like to add more fields like “country” in the Registration Page.”</li>
<li>custom new user default role</li>
<li>recaptcha on registration</li>
</ul>

Multisite User Registration Manager

Worked for me

Provides a system for registration requests and their processing in multisite. Two-level moderation.

User Spam Remover vs Multisite User Registration Manager