The RegisterThrottle plugin throttles registration by IP address

Installation
============
This plugin is enabled by default on public instances, otherwise it can be
enabled by adding "addPlugin('RegisterThrottle');" to the bottom of your
config.php

Settings
========
regLimits: Array of time spans in seconds to limits. Default is 3 registrations per hour, 5 per day, 10 per week.
silenced: Disallow registration if a silenced user has registered from this IP address

Example
=======
addPlugin('RegisterThrottle', array(
    'regLimits' => array(
                        604800 => 10, // per week
                        86400 => 5,   // per day
                        3600 => 3     // per hour
                    ),
    'silenced' => true
));