First off, the ants would probably be a plus believe it or not while they are consuming any left over honey drippings.
There is nothing long term you could spray to keep the scent of propolis away from scouts. Bees are the only insect that I know of that make the stuff, so they will want in.
Moth balls work as a fumigant. Read the box sometime, you don't want an entire box putting out vapors you could breathe in. Besides I can think of two removals I've done in wall voids where each had an entire box of moth balls in the cavity right next to the bees.
I believe in bee proofing but some buildings are almost impossible to bee proof so I bee proof the ones I can and leave the ones that can't up to the home owner and a contractor.
The ones I feel confident bee proofing I like to fill the void space (completely) with non backed fiberglass insulation and I caulk and seal. Any gaps larger than 1/4 inch need some type of backing before being caulked such as Stuff It http://store.doyourownpestcontrol.com/category.aspx?iid=335&gclid=CJC-2_a_4agCFSFl7AodnRqiIQ
And lastly, never ever under any circumstances tell a customer that bees will not come back because they will. If they can't get in they have no choice but to move on.