Ability to kill / force-stop depends on Android version and root.
Force-stop: This method prevents an app from running in the background until user opens it again. This is the preferred method to prevent battery drain, however it is not available on all versions of Android.
It requires root or 3C Companion. You can also use an accessibility service, but it may cause some ugly flickering.
To enable force-stop on a non-rooted Android device, please use 3C Companion or enable the app’s accessibility service.
Kill: While available without root on Android up-to version 7.0, it won’t prevent an app from restarting if it uses background services. This method will not interfere with other app’s behavior, but will unlikely prevent battery drain caused by apps running in background.
NB: If you have issues stopping apps, please refer to this page for a solution.