Should Giant Alliums be dead headed and if so, when and where on the stem?
I like the seed heads on alliums but if deadheading I'd remove the stem as well, for appearance not the plants benefit.
If it has finished, a gentle tug pulls the stem out of the bulb, leaving the bulb in place for next year.
The shape of the head of those types of onions is to do with seed distribution. The seed head and stalk detach from the bulb and are blown across the steppes, scattering the seed as it goes.