When it passed the Saving America's Pollinators Act in 2013, Congress directed the Environmental Protection Agency to suspend registration of several insecticides; carbaryl, which is the active ingredient in Sevin dust, wasn't one of them. This doesn't mean you should use it to kill any bees you find bothersome, but in some situations, control is a matter of safety. According to the Louisiana Cooperative Extension Service, those situations include bees that are nesting in your walls.
Toxic to Bees and Wasps
Carbaryl, the active ingredient in Sevin dust, is considered moderately to very toxic to animals and humans, so you should always wear gloves and a respirator when using it. It kills a large variety of insects, including bees and wasps, both on contact and by ingestion. In order to use it successfully for bee control, you need to apply it as close to the hive as possible. If you simply locate an entrance to the hive and use it there, the bees that pass through that entrance might not make it to the nest, and the queen and house bees will be unaffected.
Dusting the Hive
The only time you should consider killing a hive with Sevin dust is if it's accessible through an exterior wall -- it isn't safe to use Sevin indoors. Moreover, you should never spread the dust in places higher than your own chest. Before dusting, make an effort to pinpoint the location of the hive. If it's behind a wall, you can often find it by putting your ear against a glass and moving the glass along the wall until the sound of buzzing is the loudest. Bore a small hole at that point and blow in the dust directly from the container. Disturbed bees may come flying out of the hole, so wear protective clothing, gloves and a beekeeper's hat to avoid getting stung.
The Best Time to Dust
The best time to dust a hive is in the late afternoon, after the bees are done foraging and have returned to the hive for the night. Hives can last for many years, and if the one you are treating isn't new, it's best to treat it in the early spring, when the honey supply is at its minimum and the bee population is low. You probably won't get all the bees with a single application of dust -- check the hive for buzzing regularly and keep dusting until you're sure the sound has stopped.
Remove the Hive
Once you're sure the bees are dead, your work isn't finished. It's important to cut into the wall or soffit and remove the hive; it adds a load to your framing and it's a fire hazard. Moreover, a new colony may occupy it if you leave it in place. Before you decide to eradicate a nest, it's important to positively identify the insects who are using it as bees. Yellowjackets look like bees, but they are more aggressive, and dusting them can be dangerous. Yellowjacket nests last only for a single season and are never used again, so it may be best to leave them alone until winter, when they die.