Most people who edit wikis are there to help the project and increase the knowledge they contain. However, sometimes someone appears who wants to damage the wiki.
The good thing about wikis is that bad edits are easily fixed! A lot of the time it's quicker to clean up than it is to make a mess, so the good editors have the advantage.
The key to clearing up is the "history" page. Every page has one, which lists every version of the page, right back to the time it was created.
You can find the history page by going to the page and clicking the arrow next to the page's "edit" button. It may look confusing at first, but the most important parts are the dates, and clicking on one of them will show the version of the article at that time.
All you need to do is find the last good version, click "edit" then save without making any changes. And that's the vandalism gone!
Admins also have a tool to remove a vandal's edit in one click. See this help page for more about that.
And if you have too much vandalism to clean up, we have a user group called SOAP that can help! Just go to the SOAP Wiki (previously 'VSTF' Wiki), and ask for help there.
They are experts in cleaning wikis, so can help you get everything back to normal!