I think the OP finally clarified that he wanted to stop background apps for the purpose of improving phone speed (and maybe better battery life). In that sense, playing around with task manager and clearing RAM is NOT going to give you much of an effect. For example clearing RAM forces the launcher to reload the wallpapers again, so it actually takes up more juice at that point.
The only real way is to root your phone so you can specifically control which app runs and which doesn't. Don't ask me how though because I didn't root since I am perfectly happy with my phone currently.
Cheers