Spammers found my MediaWiki based website, and flooded the site with over 1.2GB of link spam. To fix this, I locked down my site, so only sysops can change it, by adding the following to the end of my LocalSettings.php file:

// Implicit group for all visitors
$wgGroupPermissions['*'    ]['createaccount']   = true;
$wgGroupPermissions['*'    ]['read']            = true;
$wgGroupPermissions['*'    ]['edit']            = false;
$wgGroupPermissions['*'    ]['createpage']      = false;
$wgGroupPermissions['*'    ]['createtalk']      = false;

// Implicit group for all logged-in accounts
$wgGroupPermissions['user' ]['move']            = false;
$wgGroupPermissions['user' ]['read']            = true;
$wgGroupPermissions['user' ]['edit']            = false;
$wgGroupPermissions['user' ]['createpage']      = false;
$wgGroupPermissions['user' ]['createtalk']      = false;
$wgGroupPermissions['user' ]['upload']          = false;
$wgGroupPermissions['user' ]['reupload']        = false;
$wgGroupPermissions['user' ]['reupload-shared'] = false;
$wgGroupPermissions['user' ]['minoredit']       = false;

// Implicit group for accounts that pass $wgAutoConfirmAge
$wgGroupPermissions['autoconfirmed']['autoconfirmed'] = true;

// Implicit group for accounts with confirmed email addresses
// This has little use when email address confirmation is off
$wgGroupPermissions['emailconfirmed']['emailconfirmed'] = true;

// Users with bot privilege can have their edits hidden
// from various log pages by default
$wgGroupPermissions['bot'  ]['bot']             = true;
$wgGroupPermissions['bot'  ]['autoconfirmed']   = true;
$wgGroupPermissions['bot'  ]['nominornewtalk']  = true;

// Most extra permission abilities go to this group

$wgGroupPermissions['sysop' ]['move']            = true;
$wgGroupPermissions['sysop' ]['edit']            = true;
$wgGroupPermissions['sysop' ]['createpage']      = true;
$wgGroupPermissions['sysop' ]['createtalk']      = true;
$wgGroupPermissions['sysop' ]['upload']          = true;
$wgGroupPermissions['sysop' ]['reupload']        = true;
$wgGroupPermissions['sysop' ]['reupload-shared'] = true;
$wgGroupPermissions['sysop' ]['minoredit']       = true;

$wgGroupPermissions['sysop']['block']           = true;
$wgGroupPermissions['sysop']['createaccount']   = true;
$wgGroupPermissions['sysop']['delete']          = true;
$wgGroupPermissions['sysop']['deletedhistory'] 	= true; // can view deleted history entries, but not see or restore the text
$wgGroupPermissions['sysop']['editinterface']   = true;
$wgGroupPermissions['sysop']['import']          = true;
$wgGroupPermissions['sysop']['importupload']    = true;
$wgGroupPermissions['sysop']['move']            = true;
$wgGroupPermissions['sysop']['patrol']          = true;
$wgGroupPermissions['sysop']['autopatrol']		= true;
$wgGroupPermissions['sysop']['protect']         = true;
$wgGroupPermissions['sysop']['proxyunbannable'] = true;
$wgGroupPermissions['sysop']['purge']         	= true;
$wgGroupPermissions['sysop']['rollback']        = true;
$wgGroupPermissions['sysop']['trackback']       = true;
$wgGroupPermissions['sysop']['upload']          = true;
$wgGroupPermissions['sysop']['reupload']        = true;
$wgGroupPermissions['sysop']['reupload-shared'] = true;
$wgGroupPermissions['sysop']['unwatchedpages']  = true;
$wgGroupPermissions['sysop']['autoconfirmed']   = true;
$wgGroupPermissions['sysop']['upload_by_url']   = true;
$wgGroupPermissions['sysop']['ipblock-exempt']	= true;
$wgGroupPermissions['sysop']['DeleteOldRevisions'] = true;

// Permission to change users' group assignments
$wgGroupPermissions['bureaucrat']['userrights'] = true;

Next, I backed up my MySQL database with

mysqldump --opt db_name >db.sql

And deleted all the spam, by running the following mediawiki_purge.sqlscript:

DELETE FROM user WHERE user_id NOT IN (1,2,3,4,6,81);
OPTIMIZE TABLE user;
TRUNCATE TABLE archive;
OPTIMIZE TABLE archive;
DELETE FROM recentchanges WHERE rc_user NOT IN (SELECT user_id FROM user);
OPTIMIZE TABLE recentchanges;
DELETE FROM revision WHERE rev_user NOT IN (SELECT user_id FROM user);
OPTIMIZE TABLE revision;
DELETE FROM text WHERE old_id NOT IN (SELECT DISTINCT rev_text_id FROM revision);
OPTIMIZE TABLE text;
DELETE FROM interwiki;
OPTIMIZE TABLE interwiki;

via

mysql db_name < mediawiki_purge.sql

As a result, the size of my MySQL dump file has dropped from 1.2GB, to only 8MB.