On older Android versions I used to use Advanced Task Killer (by Rechild, couldn't find it in the Play Store anymore) to kill off unused programs, but I started it manually only, as I didn't want it to kill apps 'under my fingers'. I think I stopped using that around Android 5 or 6, but it should still work on those versions. AFAICS, killing an app is the only way to close it's (remaining) views.