Flypaper works. It is ecologically sound and simple. Flies like the sticky gunk, but get stuck.
The problem - one must change the flypaper or else look at unsightly fly caracasses hanging around for days and weeks. Old flypaper can lose its effect.
I propose a flypaper machine. It could be
a windup machine for simplicity - one could use a clock mechanism. A refillable reservoir is full of goo. The goo is deposited atop a roll of paper which slowly unrolls with the motion of the machine. Flies land in the goo and are stick. The dead flies move with the slowly flowing goo and paper into a bag or recptacle below.
This would accomplish several ends.
1: Fresh goo - In a sandy or dusty environment, exposed goo may lose its stickiness in a few days. In the reservoir, goo stays fresh.
2: Cleaner - long, tacky flypaper strips are hard to move about and invariably stick to other items in the environment, like your hair.
3: Adaptable - one could use stink goo to attract bluebottles or houseflies, fruity goo to attract fluit flies and so on.
4: Ecological - no chemicals except benevolent goo.