I know one can slap/swat, one-handed in mid-air, a fly dead: I've done it. I don't know if it died when it hit my hand/fingers, or when it hit the ground, but it was alive before and dead after. It was a large-ish fly, though, body maybe 6mm or quarter inch in length. I don't ever recall the same happening with much smaller fly-like bloodsuckers like mosquitoes (culicadae), black flies (simuliidae), and no-see-ums (ceratopogonidae), which I grew up swatting in the hundreds or thousands each summer (North of the Arctic circle); one needs to either slap them against something, or grasp them and crush between fingers, or they'll just continue flying.
But punch, with ones fist, fingers curled? Guinness' record for the fastest punch was apparently clocked (pun intended) at 45 MPH, so I do believe it is possible for the larger fly variants, but not the smallest ones like fruit flies.