@BBH was invited to speak at the ACTBKA tonight and provided answers during the Beginner’s Corner segment. One of the questions asked was about robbing and approaches that can be used to prevent, or at least limit the amount of robbing occurring.
The basic approach is to reduce the hive entrance by using an entrance reducer, or by blocking off part of the entrance to reduce the amount of space that the guard bees need to protect. With the space reduced it will both limit the rate at which potential robbers can enter the hive and gives the guard bees a greater chance of obstructing the entry of robbers. If top entrances are also configured in the hive that give direct access to the supers, they should be closed completely forcing entry through the reduced main entry.
Another method that was discussed in the response from @BBH involved placing a piece of glass over the entrance. Without a diagram, it may not have been clear exactly how the glass was placed to obstruct the entrance, so I have created an illustration to hopefully clear up confusion.
In this method a piece of glass is placed leaning against the hive in front of the entrance. The intention is to deter opportunistic robbers that are interested in quick access to the hive. The bees that are from the hive will spend longer determining how to return to the hive by navigating around the edges of the glass. As was also discussed, this method can also be used to deter European Wasps that attempt to infiltrate hives at this time of year.