There is a small possibility that if you cut it off now, it may form more buds before the end of the growing season. If you're harvesting them to eat, you generally cut a couple of inches down the stem from the bottom of the bud; I don't see any reason to do it differently in your situation.
On the other hand, the plant has already put energy into growing this flower so you could just enjoy it while it's in bloom. It will take some energy to maintain the flower, but artichokes are vigorous growers in the right conditions, so I doubt it will hurt the plant to leave it. My neighbors leave theirs to flower and they keep coming back year after year. If you do leave the flower, you should remove it once it starts to wither, so that the plant won't try to put more energy into making seeds.
However, I think the single best thing you could do for that plant is to put it in the ground. Artichokes develop long tap roots, typically growing as deep as the plant is tall, so if it's being confined to that pot, it's never going to reach its full potential.
