You can make a Robbing Screen very simply. You don’t need to buy/order anything. Bend some screen wire (window screen will work) in the shape of a “W” and staple it across the reduced entrance. Extend it 3-4 inches or more on either side of the opening. Install it early in the morning before the bees come out or late in the evening when the bees are in. Install it immediately in order to stop robbing, don’t wait!! The bees that live there will figure out how to get back in, the bees that want to rob the hive will try to go straight in and get stopped by the screen.
I attached some pictures of what I mean.