Bananas used in drink recipes don't have to be spot free unless they're used as a garnish. If the banana is an ingredient, a ripe, slightly spotted banana is preferable.
Just store your bananas on the counter, or hang them from a hook. Don't put them in the refrigerator! If you have bananas that are about to become over-ripe and spoil, seal them in a plastic bag and put the in the freezer for later use.