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:
<ul>
<li>
Automatically deletes user registration spam and other orphaned, never-used accounts.
</li>
<li>
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.
</li>
<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.
</li>
<li>
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.
</li>
</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.

Scott Reilly

This plugin allows you to restrict the usernames that new users may use when registering for your site.
If open registration is enabled for your site (via Settings -> General -> Membership (“Anyone can register”)), WordPress allows visitors to register for an account on your blog. By default, any username they choose is allowed so long as it isn’t an already existing account and it doesn’t include invalid (i.e. non-alphanumeric) characters.
Possible reasons for wanting to restrict certain usernames:
<ul>
<li>Prevent usernames that contain foul, offensive, or otherwise undesired words</li>
<li>Prevent squatting on usernames that you may want to use in the future (but don’t want to actually create the account for just yet) (essentially placing a hold on the username)</li>
<li>Prevent official-sounding usernames from being used (i.e. help, support, pr, info, sales)</li>
<li>Prevent official username syntax from being used (i.e. if all of your administrators use a prefix to identify themselves, you don’t want a visitor to use that prefix)</li>
<li>Prevent spaces from being used in a username (which WordPress allows by default)</li>
<li>Require that a username starts with, ends with, or contain one of a set of substrings (i.e. “support_”, “admin_”)</li>
<li>Require a minimum number of characters for usernames</li>
<li>Limit usernames to a maximum number of characters</li>
</ul>
When attempting to register with a restricted username, the visitor will be given an error notice that says: 
ERROR: This username is invalid. Please enter a valid username.
NOTE: This plugin does not put any restrictions on usernames that the admin chooses for users when creating user accounts from within the WordPress admin. This only restricts the names that users choose themselves when registering for your site.
SPECIAL NOTE: Many membership plugins implement their own user registration handling that often bypasses checks (and hooks) performed by WordPress. As such, it is unlikely that the plugin is compatible with them without special plugin-specific amendments.
Compatible with Multisite and BuddyPress as well.
Links: <a href="http://coffee2code.com/wp-plugins/restrict-usernames/" rel="nofollow ugc">Plugin Homepage</a> | <a href="https://wordpress.org/plugins/restrict-usernames/" rel="ugc">Plugin Directory Page</a> | <a href="https://github.com/coffee2code/restrict-usernames/" rel="nofollow ugc">GitHub</a> | <a href="http://coffee2code.com" rel="nofollow ugc">Author Homepage</a>
<h3>Hooks</h3>
The plugin exposes one filter for hooking. Typically, customizations utilizing this hook would be put into your active theme’s functions.php file, or used by another plugin.
c2c_restrict_usernames-validate (filter)
The ‘c2c_restrict_usernames-validate’ hook allows you to add your own customized checks for the username being registered. You can add additional restrictions or override the assessment performed by the plugin.
Arguments:
<ul>
<li>$valid (boolean): The assessment by the plugin about the validity of the username based on settings. True means username can be used.</li>
<li>$username (string): The username being registered.</li>
<li>$settings (array): The plugin’s settings.</li>
</ul>
Example:
<pre><code>/**
 * Add custom checks on usernames.
 *
 * Specifically, prevent use of usernames ending in numbers.
 *
 * @param bool $valid True if the username is valid, false if not.
 * @param string $username The username.
 * @param array $options Plugin options.
 */
function my_restrict_usernames_check( $valid, $username, $options ) {
 // Only do additional checking if the plugin has already performed its
 // checks and deemed the username valid.
 if ( $valid ) {
 // Don't allow usernames to end in numbers.
 if ( preg_match( '/[0-9]+$/', $username ) ) {
 $valid = false;
 }
 }
 return $valid;
}
add_filter( 'c2c_restrict_usernames-validate', 'my_restrict_usernames_check', 10, 3 );
</code></pre>

Restrict Usernames

Restrict the usernames that new users may use when registering for your site.

User Spam Remover vs Restrict Usernames