Hi Pedro,
It would appear that the bar has a problem with fruit flies. Fruit flies absolutely love the products sold in a bar such as beers cider and sweet smelling spirits.
The best preventative measure is to ensure that the bar area is always cleaned and sanitised to a high standard at the end of every shift (especially the drip trays) and that all cleaning clothes are removed from the bar area and washed or disposed of. If you use bar towels/runners throw them away and use washable bar trays. And, finally, implement a thorough cleaning schedule to be followed by staff at the end of the evening's trading session.
There are numerous ways of getting rid of bar flies if it is a small infestation; try any of these:
http://www.stretcher.com/stories/03/03jul21b.c fm
Fly sprays are not effective and shouldn't be used around the bar area (taint product and glasses). If the bar has a serious infestation it would be wise to use one of the pest control companies who can solve the problem and offer far better advice than myself!
I realise that pest control may be a more costly option, but it is a small price to pay in relation to the cost of losing customers and the damage to the bar's reputation.
Best of luck