By default Mediawiki turns on the “nofollow” in all references so spam is pointless, but that doesn't seem to stop anyone.
This site has some spam blocking suggestions from Bryce Harrington
Disabled editing unless user is logged in
Protected some of the 'default' pages (e.g. Help:Contents) that spammers were hitting. Deleted others.
Banned users that had set up accounts but were just spamming.
'Help:Contents' is now protected due to excessive spamming
Added the SpamBlacklist extension. This prevents any editing from a long list of domains that have spammed Wikipedia in the past.
Blocked open proxies. Wiki spammers often use open proxies to cover their tracks. This blocks all such IP addresses based on the SORBS DNSBL.
Blocked div
CSS tags. Many spammers these days are hiding their spam in style sheets.
If you see spam report it to Stephen Hemminger or one of the other operators