We've had a tennis racquet shaped fly zapper for a couple of years (from Aldi, I think, though they are commonly available). Runs on 2 AA batteries. The idea is to get a fly to touch the electric grid which replaces the racquet strings while you keep the button on the handle pressed. It kills them instantly, then you can flick them outside or down the loo. The hard bit is actually catching up with them but if they're in the window it's easier. It makes a high pitched whine which I can just about hear. Mrs T can't hear it, but my 4 year old grandson can hear it and says he doesn't like the sound.
It does work on wasps, though I'm more likely to try to get them to leave as they're beneficial for the garden. It's a bit brutal as they're pretty tough but at least if you can stun one you can get it outside and let it fly away. It seems that if you're that way inclined you can zap them until they're trailing smoke and they'll still try to get back to base.
I felt bad after zapping a hoverfly recently as you almost never see them in the house. Silhouetted against the window it just looked like an ordinary fly.
