Wikitravel uses a spam filter to block external links to certain websites on a keyword basis. The spam filter consists of two lists of regular expressions against which edited articles are compared. If the edited text matches the regular expression, the user is warned and the file is not saved.
The main list of regular expressions is available at http://wikitravel.org/bannedcontent.txt. It is a copy of the BannedContent list on CommunityWiki, which originated the idea of a distributed regular-expression list. This list is not user-editable.
A secondary, local list of banned hostnames is maintained at Wikitravel:Local spam blacklist. This list is editable (if you're a registered user), so please add any sites that spam Wikitravel to this list.
Since this spam filtering system was put into place, we've had a very steep decline in the number of spam postings on Wikitravel. This has freed up people to work on other things, like making a great travel guide.
There are a few problems with the banned content list, as it stands:
Sometimes spam sites slip through the filter, and sometimes valid sites are incorrectly blocked — please report both kinds of problems on the spam filter talk page so they can be fixed. Alternatively, for URLs that are being incorrectly blocked, Wikitravel maintains an editable Wikitravel:Local spam whitelist. If a URL is being blocked that should not be, list it on this page and also provide an explanation of why the URL needs to be whitelisted on the whitelist talk page.