Or should they just be yanked? The flower5s were so lovely I just couldn't harvest them right away.

For one thing, each of those seed heads will provide enough seeds for probably a few hundred baby leeks. They often self-sow with wild abandon, look for little lawn-like patches of baby leeks to emerge in the spring.

They also regrow from the original bulb. In another roughly 6-8 weeks, one to four or five little baby leeks will emerge from the base of the old leek. They will be big enough to use yet this fall as small leeks, and will become full sized next spring. Once the flowers fade and start to develop into a seed head, you can pull or dig the entire plant if you want to. You will see the small cloves without a tunic (papery skin) clinging to the old stalk, attached at the basal plate. They look like peeled garlic cloves. Each of those will grow into a new leek. You can dig these, separate, and replant individually if you want them to get bigger. Left on their own to clump they will still be OK, but as large as if separated.

Finally, you can also use that little clove as is as a garlic/onion substitute. Just wash, cut off the basal plate, and slice, dice, put through a garlic press, whatever.

As Flora_UK wisely said, you can always use the old stalks as a "bouquet garni" to flavor soups and other dishes, just remove before eating (unless you're a porcupine or beaver that enjoys woody stalks).

So, yes, there is plenty of life and usefullness in a "spent" leek.