From your description, you have a standard skimmer style overflow.
You should be able to adjust, move up/down, the box inside the tank. This box is what will stop your flow, when the pump stops returning water from your sump.
As you will always have water in the, inside tank and outside tank boxes, your "U" tubes will still be full of water as well. That is why, when the return pump starts running water back to your tank, the overflow will restart, without having to do anything....ain`t nature grand

Your return line will need a way to stop it`s flow, back, through the pump, into your sump when the pump is not running..theres` that nature thing again.
If using a spray bar or nozzle you could mount it at the depth, in the tank, you want the "backflow" to stop. You could put a hole in the return line to stop the siphon effect as well. It does not need to be a very large hole, 1/8in, would work. If you go with the hole, as part of your cleaning routine, you need to be sure that hole is not gunked up, shouldn`t be with a constant flow out of it, but just to be safe..