Manual:$wgRateLimits

Details
Simple rate limiter options to brake edit floods. Maximum number of actions allowed in the given number of seconds; after that the violating client receives HTTP 500 error pages until the period elapses.

e.g.  for a maximum of 4 hits in 60 seconds.

This option requires memcached. Also, note that it is experimental and likely to change.

Default value
array(    'edit' => array( 'anon'  => null, // for any and all anonymous edits (aggregate) 'user'  => null, // for each logged-in user 'newbie' => null, // for each recent account; overrides 'user' 'ip'    => null, // for each anon and recent account 'subnet' => null, // ... with final octet removed ),    'move' => array( 'user'  => null, 'newbie' => null, 'ip'    => null, 'subnet' => null, ),    'mailpassword' => array( 'anon' => NULL, ), )

Note that the 'mailpassword' array was added in MediaWiki 1.7.0.