Bananas emmit a gas calle ethyline which is what causes them to ripen, it is produced during warm conditions and escapes into the atmosphere through tiny pores in the banana skin, that is why they go spotty... Bananas are good to ripen other fruit too by sealing the two together in a polythene bag and leaving for a period of time dependant on conditions Tomatoes react well to this treatment.
When bananas are imported they are green, and they used to be stored to ripen in banana houses filled with ethylene gas, i don't think they do this anymore tough.
The answer to the question therefore is to keep the bananas cool (they don't like fridges though (they turn black) however, once to ethylene process has started it is hard to stop.
hope this is helpfull to you.
Pete