See if you can find out what they are spraying with (all of it). Some pesticides do not affect honeybees.
this is a popular list and their effect:http://ohioline.osu.edu/hyg-fact/2000/2161.html
Is it spray or powerder application. Spray is better for the bees, since the spray will dry and be less harmul to bees.
Do not seal up the hive completely. You will want to close up all entrances so the bees can't get in or out. To close up is to stll allow air to get in and out of the hive. I have taped screen over all of the entrances, propped the outer cover to help with ventilation. You will want to do this just after sunset, when the bulk of the bees are back in the hive.
When you need to take the screens off. Wear your beesuit, because they will be very angry after being locked in for a while.