91% isopropyl alcohol with a trigger sprayer. A fine mist kills fruit flies,
and set on stream it works great for houseflies that have landed on a surface.

I also used the alcohol to remove pen ink from clothing that accidentally got
washed/dried with a ballpoint pen. It was very time consuming, but did work.
My wife was sure that her clothes were ruined, but I soaked each stain and
rubbed, and sprayed alcohol point blank through the stain, the repeated over
and over, and then sprinkled on a little phosphate (STPP) and rubbed that in
before hand washing in a very light chlorine bleach solution, and hand rinsed.
It was very labor intensive, but much loved clothing items were salvaged, and
I felt quite the Heloise.

--Bryan