Do not put bananas in the refrigerator unless you want to ruin them. Although they much prefer fruit, bananas being something of a favourite, fruit flies feed on the most surprising things when nothing else is available. At the right stage, they will go for potatoes, onions, etc. The advice so far to check for every scrap of vegetable matter and deal with it is good advice. There are smells which deter fruit flies (peppermint, eucalyptus - google them) and planting these around wherever they might settle would wok to a considerable degree. It has always seemed to me that fruit flies are attracted to the smell of vinegar but maybe I am wrong.