Disclaimer: I am just an amateur, get professional advice.
If you have two DC sources it would be easy, you just add a one way (diode) and power/current can only flow in one direction.
You might be able to simulate this by feeding grid into UPS(double conversion AC->DC-AC) and then having your grid-tied inverter connected to the output of the UPS.
Thus if you draw low power from the UPS output the grid-tied inverter will elevate the voltage slightly but nothing will flow back through the UPS AC->DC
It might also provide the additional benefit of extending the runtime of the ups, and allow the grid-tied inverter to operate when the mains power fails.
By connecting the grid-tied inverter to the output of a double conversion UPS you have effectively removed it from the grid.