It is best to contact a beekeeper to remove a swarm. Beekeepers use a box to scoop up the bees and take them to another location. Bee swarms, called clusters, generally leave when they locate a new nesting site, making removal unnecessary.
Swarming bees can leave behind a small amount of wax, which can attract new swarms. If a swarm does not leave and the number of bees appears to dwindle, they may be nesting inside a wall. Exterminating bees inside a wall is done by placing mesh over the hole, so they cannot leave to forage for food; however, being enclosed might cause them to chew a hole into an interior room. Bees that enter a room typically attempt to fly toward windows, and they often die trying to escape.
Beehives established inside a wall can be enormous and can contain large amounts of honey. While getting an exterminator to apply pesticide can be effective, it leaves the bodies and honey behind, which requires professional removal using protective clothing. Once the bees and honey are removed, seal all holes in the building to prevent another swarm from entering. This is important because bees have a keen sense of smell and might be attracted to the odor of the old hive.