What can be done to prevent wasps from coming into the attic if no one will spray an unfinished attic?

Make sure you seal all outside openings: Caulk or glue all cracks and seams, and place screens across vent openings. If wood components are found to be bored through (Bumble Bees will chew a hole right through)treat holes with creosote and apply a sealant over it to prevent bleeding into the paint then oeverpait to match original color.