I have used CO2 to disable flies (they just go to sleep) so that I can put them in a venus flytrap. Although, in my case, I was doing this in a science lab so I had access to a specialized device (a porous marble stone with CO2 being diffused through it). Once they're disabled, you can put them in the trap, trigger the little hairs, and when the fly wakes up, the trap stays closed due to the fly wiggling around, and a month later they open up and a dessicated fly falls out.