Using a fly spray for one or two flies isn’t an issue, but for an infestation, taking things into your own hands is risky.
If you have a food business, you can’t just spray or put down random chemicals all over your premises because you could contaminate your product, and who really wants fly traps with countless dead flies lying around in their workplace?
The only effective way to get rid of a fly infestation and prevent them from coming back is calling in professional pest control.