FarmVille is a gaming application for Facebook that lets users construct a virtual farming community with their friends. If the game begins to take up too much of your time, or if you're just not interested in playing the game anymore, FarmVille is easily removed from your Facebook account. Once you delete the FarmVille application, the game immediately disappears from your profile and old FarmVille score updates are removed from your wall.
Instructions:
1) Log in to your Facebook account and click the white "Account" link in the top right corner of any screen on the site.
2) Choose "Account Settings" from the "Account" menu.
3) Select the "Applications" tab in the left column of the screen.
4) Locate FarmVille in the list of applications. Your applications are listed by date of last use, with the apps you used most recently at the top of the page.
5) Click the "X" to the right of FarmVille's name. Click the blue "Remove" button in the pop-up window and then "Okay" when prompted.
Source: eHow