You may occasionally need to Force Stop an app. This is primarily done when an app has Services still running in the background, which can happen (and often should happen) even if the main app is not currently running. This page provides instructions for doing so on several different Android versions. There are many different Android versions and devices so the instructions for your particular device might be slightly different, but it will hopefully be similar.
(Note: These instructions use the Triple Town app as an example, but this technique should allow you to Force Stop any app.)