Any idea what they are using to spray?
Most pesticides are extremely toxic to aquatic organisms including fish. If they are using something like Bifenthrin (it's banned in most countries but still gets used by some exterminators in the US, and it's not banned at all in Australia), then it is lethal to fish and also kills everything in the animal kingdom, including people. It leaves a carcinogenic residue behind for at least 5 years in outdoor areas, and 10+ years in sheltered areas.
If they use pyrethrin or pyrethroids, they are lethal to fish too, but safer for people. However, I don't trust anything that isn't fit for human consumption when it comes to safety.
TNG pretty much covered it. Stop air and anything being sucked into the tank by covering it in plastic or whatever you can. Ask the exterminator to be careful with spraying.
If you have fish buckets, hoses, nets, etc, put them somewhere safe during spraying (maybe in the car).
Try to avoid the pesticide too because there are no safe ones.