Helen's flytrap, made with a large plastic bottle, works best if you arrange some sort of shade over the inverted bottleneck so that they can't see a way out there. Perhaps a good excuse for buying a cocktail with a wee umbrella in it? Fishheads work fastest, and a wineglassful of soapy water kills them quickly. Suffocation, I suppose.
There's a fiddle tune in Oz called 'The Blowfly.'