Open your app, and navigate to Settings. Tap on the account you want to remove, and from the options press ‘Remove account’. After removing Facebook from your, the app will ask Facebook to remove it’s access to your account. Please note that this will remove all of the account’s data from your library. 

You can also do this manually by accessing Facebook Apps Settings. Selecting 'Apps and websites' will show which applications have the ability to access your Facebook data. Checking the box next to '' will give you the option to remove's access to Facebook.