I won’t bore you with my rants on how silly I think the FarmVille application on Facebook is because you undoubtedly found this post while looking for a way to block the application.
(Side note: This method mentioned here works for any Facebook application actually, not just FarmVille)
So here is how it works:
Visit the FarmVille homepage on Facebook: http://www.facebook.com/apps/application.php?id=102452128776
Under the FarmVille logo image on the top left of the page, you will see a few links (see picture below). The last link should say something along the lines of: ‘Block Application’. Click this link! In the image below, you will note that mine actually says: ‘Remove Block for Application’. So if you choose, you can always remove the block at a later time.