Posted: Today at 13:05
Pick them young, not like they are in the supermarket, those are too old. Pick them before they have to have the strings removed.
Not sure what you mean by 'prune'. Just pinch out the growing tip when they threaten to get too tall to reach the beans.
Usually we have a frost which kills them about the same time you get tired of eating them. Try not to freeze them, they should be treated like a seasonal vegetable IMO. But above all eat them eat them young.