In my experience, after flowering there isn't as much flavor in the leaves. So if you're growing for the purposes of harvesting, pinch the flowers off.

Water regularly and start pinching back frequently! Once basil is allowed to flower it has pretty much fulfilled its destiny and will stop producing much in the way of additional foliage. On the other hand, if you are not into making pesto or seasoning your food, the bees will love you for leaving them the flowers to visit.

Cut them off as soon as you see them. They plant uses all there "strength" on the flowers, so the plant stops growing. I have a basil plant, I cut all off the flowers off, and it's growing well.