To revert a page to a previous revision: use the revisions links at the bottom of the page to find the highest revision number that did not contain spam. Be careful, because sometimes the spammers will add spam links to a page instead of completely vandalizing it, so be sure to scroll down the page and look for any spam links that were added. 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 "Summary of changes" field, and put your name in the "Your name" field. Hit save, and all will be well.
To delete a page which was created by spammers, edit the page and remove all the content. Deleted pages get put in the Deleted articles list with their history preserved, and can be restored later.