Extension:UserThrottle

What this extension does
Monitors global frequency of user account registrations and throttles apparent flood attacks, regardless of origin. Not always suitable for use on large public sites, but it's been used during occasional particular vandal attacks on Wikimedia in the past.

Installation
To install this extension, add the following to LocalSettings.php:

Configuration
You can adjust the throttle settings in $wgGlobalAccountCreationThrottle:

If you legitimately have signup floods, you may wish to adjust these.