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