What is wiki spam? 
WikiSpam is the wiki equivalent of e-mail and blog spam. Wiki spammers edit pages to include advertising links to their sites. Spam links appear as full URLS (e.g. http://example.com), as named links with a spam keyword, or as numbered links (e.g. ). Often a spammer will completely replace the legitimate content of a page with their spam, and may add many different links, with a range of URLS and keywords.
Other nasty spammers (such as spambots) will edit a mass of pages in few minutes, even replacing good links with bad links such as vandalizing article references.
Why are the spammers doing this? 
Spammers are not really hoping that you will click on their links, although if you do they are likely to be happier. Their actual goal is to improve their position in Google searches. Wikis are characterized by their open access and willingness to let anyone edit pages. Google measures its PageRank based on links from one site to another, weighted by the PageRank of the site linking to the other. Wikis are PageRank machines, being both massively linked and with hundreds or thousands of pages. These two factors, openness and PageRank, make wikis a tempting target for spam attacks.
For most new installations of mediawiki software spamming is pointless, because external links have the rel=nofollow attribute, which means they will get no google ranking boost from the wiki at all. However, many spammers seem not to realize this, and carpet-bomb all wikis they find anyway.
How to remove spam 
Anyone who can edit the wiki can manually revert abusive edits. MediaWiki (and all good wiki software) allows you to overwrite the current (spammed) version of a page with an older version.
- Click "History" tab at the top of a page
- Click on the link of the revision you want to recover (the date) This will take you to a page with the text of the old revision.
- Click "Edit" tab at the top.
- Add a summary, such as "revert spam" and click the 'Save' button.
When you revert a change, the previous spammed version of the page is still available under the "History" link. As a user, there is nothing you can do about this. It has to be this way, because otherwise vandals could do malicious reverting of changes.
If you are an administrator you may use the rollback tool to quickly revert edits.
If a spammer creates the first version of the wiki page (usually with a spam related page title) then you should follow the procedures on that wiki for deleting a page. If there are no such procedures established, you might replace the page text with a redirect such as: #REDIRECT [[Wiki spam]] This will help avoid google indexing the spam. An administrator will need to carry out the actual page delete.
Watching out for Wiki Spam 
Always use the recent changes display to watch for spam. It is not enough to just look at the important pages of your wiki. Spammers have been known to find sneaky ways creating spam pages on a wiki in places where they may go unnoticed. This is particularly a problem on MediaWiki software (see MediaWiki Default Pages Spam), but all edits will appear on 'Recent changes' unless you are hiding minor edits. Go to preferences on your wiki, and under the 'recent changes' settings, check you are not hiding minor edits.
If you haven't checked 'recent changes' in a long time, then you may need to show the last 500 edits in the last 30 days. Click on the links at the top, or edit the page URL to set this time limit to even longer.
Other action users can take 
Report the problem to the linked website
Go to the linked website. In fact you should go to the top level domain name of the linked URL (Note: Use a fully up-to-date secure browser, and take care not to click 'OK' on any dubious looking messages) You may find that website looks reasonably innocent, and often it is! These days the vast majority of spam links are pointing, not directly at a spamvertised site, but at a spamdexing link farm and/or redirecting HTML. Spammers often set up this kind of HTML on free web hosts, portal hosts, bug-tracker attachments, on hacked webservers, or using a 'shorter URL' service. This means the people running the website (visible at the top level domain) are not spammers! They are just unwittingly helping the spammers by hosting their site. This is very common nowadays. The good news is, if you can find an email address or contact form, and report the problem (be polite, supply them with the spammy URLS, and with google searches revealing them), there's a good chance they may fix the problem. This is a great way to fight back against the spammers. You may be damaging their spam operations across many websites.
Clean spam from other wikis
There's lots of other wikis which are having the same problem. You can use google to find them. A search for "mediawiki viagra" will usually turn up some examples. You could revert spam on other people's wikis. This isn't just an altruistic act of kindness to fellow wiki users, it actually helps you (and everyone) by helping to discourage wiki spamming. Less wiki spam on the internet in general, will mean that wikis are a less attractive target for spamming. You can even target a particular spammer who has been annoying you. Search for their domain names/keywords and undo their spamming! This is something wikis are uniquely capable of (you can't clean up link spam on somebody elses blog or messageboard in this way), however you will often be forced to create user accounts on the other wikis in order to do the reverting edits.
Create pages to explain the problem
Create a page called [[WikiSpam]] or [[Wiki Spam]] to explain the problem, and encourage your users to revert spam as soon as they see it. Feel free to link to this page, or copy and paste information from it, especially the top three sections.