Answer: Actually, it is best to keep basil from flowering because, as an annual, once the plant flowers and sets seed it will go into decline.
Usually gardeners encourage bushiness by a process called pinching. They begin literally pinching off or trimming off the growing tip(s) of the branches when the plants are quite small. (You can eat the pinchings!) They repeat this several times until the plants are as bushy as desired.
When the plants become quite dense, some gardeners simply trim or shear them regularly and use the shearings in cooking. Other gardeners will cut off a larger proportion of the plant for harvest, especially if they plan to dry or freeze a quantity of basil all at one time. These more drastic harvests can be done just a few times a season as they are stressful on the plants and eventually the plants just "wear out". In either case, the trimming prevents the plant from flowering and it regrows in order to try to flower.
Since your plants are already quite tall, I would suggest cutting them back by about a third. This will give you a nice harvest and allow you to begin pinching as it grows back. When you do this, be sure the plant receives adequate water and nutrients to regenerate itself. Enjoy your basil!
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