Here is a hive in the process of being robbed by strong colonies. You can see the fighting and the frenzy.
What to do? Keep strong colonies, if you catch it you can put a sprinkler on top to discourage bees from coming in. Robbing screens and reducing the entrance can help.
On this hive I put on a robbing screen, taped a bee escape on the hole in the front, and hosed off the box several times (do you think I could find the sprinkler? Nope). It seems to have worked but the robbing bees were pretty determined.
