To revert a page to a previous revision: use the History link at the bottom of the page to find the latest article revision that does not contain spam. Once you're at the last non-spam revision to the page, hit the "Edit" link at the bottom. Fill in some pithy diatribe against spam in the "Edit summary" field. Hit save, and the article will be restored.
To delete a page which was created by spammers, you need to be logged in (you can obtain an account on the registration page). When you are logged in, there will be a Delete button next to the Create button at the bottom of the page.
To watch for spam, see the page Keeping up with CLiki