Wasps are the type of insects you don't want in and around your home because they tend to be somewhat aggressive when it comes to protecting the group. If you discover a wasp nest in a hard-to-reach location, such as enclosed stairs, you can still get rid of the nest of insects, but you will need to use your observational skills and a little patience.
Sit at a distance and watch the staircase to locate where the wasps are entering. The best time to observe is in the evening when the wasps start to settle down.
Drill a hole in the staircase if necessary to widen it to feed the spray tube into the entrance. The insecticide usually comes with a little tube to insert onto the nozzle to direct the spray. If you need to widen the entrance hole, do it in the afternoon, when most of the wasps are out of the nest. Do it quickly. If the wasps show up, leave and come back later.
Wait until evening and feed the tube of the sprayer into the entrance hole. Spray the wasp insecticide. Try to aim the tube toward the nest.
Wait a few days, and then begin to watch the staircase again. If you see wasps, use another can of wasp spray.
Seal up the wasp entrance hole once you no longer see any wasps going in and out. Fill with an expanding foam and let it harden. Sand it smooth when the foam dries.