thats easy. On the return hose from the pump in your sump going up to the return in your tank above you would install a "check valve". Home depot sells them but I prefer the ones that lfs sell because they are clear and you can inspect them. In addition, drill a 1/16 hole at the bottom of the elbow pvc fitting that hangs in your tank on the return, this is called a siphon break and it does exactly that. In the event that you check valve gives out or is stuck that little hole is enough to stop the water from back siphoning. The other thing to keep in mind is that you will only drain as low as your return, so if you have a spout only an inch below the water line, the water would drain only up to that point (provided you dont have a check valve and shiphon break).
Let us know what else you need to know.