I kill them with a spray bottle filled with 70% isopropyl alcohol. The mist is difficult to avoid, and once they’re knocked down, I drown them with a few more blasts. I think the surface tension of the alcohol is able to permeate the tiny holes (spiracles) that would normally keep water out of their respiratory system. No guts, just wipe away the fly’s body and any excess alcohol. Anything you miss will simply evaporate. Bonus: 70% isopropyl alcohol is a disinfectant, so you’ve already helped clean any disease the fly may have been carrying.