If you mean to back up the ATC so that it only feeds in one direction, just put a prussik on the line and feed rope through ATC or prussik.
If you want a minimalist approach with only a crabiner just use a munter hitch, it works really well. I met some Germans who use it to belay sport lead, I wouldn't go that far but for belaying from the top its fine.
If you want to block off the ATC completely, lets say in the case of a fallen climber and you need to get the load off or do some work, you feed the rope through the carabiner and itself. A bit hard to explain I think its called a munter mule knot, if you do a search for knots on the web you will find it.