I take it you are talking about fruit flyies, tiny little midge type flies.
if you are the only reason they are there is because you are not cleaning well enough behind the bar, as they feed on the beer that have turned to sticky syrup which you will find in every crevice behind that bar. the only thing you can do is scrub and make sure that you wash down properly every day on and under the shelves and especaily around the pumps and glasswash area
you dont pay your staff to stand around chatting get them scrubbing lol
And I am by no means saying you are clean, you are just not beer fly clean, I ran pubs and resturants for 15 years and they only appear if the staff are slacking on their cleaning.