Android Mobile Task Management

I am looking for a Task Manager for an Android phone.

I have a few apps like RetailMeNot etc etc.  When I am done with the app, I close it but it continues to run in the background.  I don't want this because it makes the phone run slower.

Two Norton Chat reps have told me to Settings - Apps then slide to "running", choose an app and force a stop.  This works, but the apps come back and run in the background again.  I don't wanna have to uninstall and reinstall each time.

I have seen Task Management apps on Google Play, but they are battery intensive, full of ads and try to get me to download more stuff.  I just need something that will keep the apps closed until I choose to open it again.

Hey Norton, PLEASE develop this for your customers so we can keep our phones in good working order like our computers.

Thomas Sandifer