Depends on the overflow. If a HOB
style and it's set up properly, it will simpley lose it's siphon once the water level goes below the lip of the overflow. Same if this is a built in overflow, the level of the water falls below the top edge of the overflow, the water will stop entering the sump.
You also need to ensure the return lines from the sump have back siphon holes drilled into them. That will break the siphon and prevent water from entering the sump as well.
Just be sure you do a few tests shutting of the power to the pump to see what happens. Also mark on the sump where the max/min water level should be. Never fill a sump more than Â½ full but a few tests should give you an optimum safe level.