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