[Hacksaw]

Top the flowers out (cut the stalk off) after they bloom.

They should not be pollinated?

[Alas Babylon!]

The idea is to force the plant to put all it's energy into producing leaves, not offspring. The same thing (pinching off flower buds) is done to basil, spinach, etc; those plants that we grow for the leaves. Unfortunately, once a plant starts to go to seed, less energy goes to the leaves, and they become bitter.

[Smokin' Joe]

I should have specified after the blooms are open, usually about a week. The cut should be made above the last leaves at the top. If you want, allow a couple to polinate, then secure paper bags about the blossom head. The seeds are incredibly small, so get a good bag. plastic can work, but make some cuts (for ventilation) near the top to keep from cooking the flower top.