Care for Allium

What do I do when the flower is finished and turned into a seedlike ball?

Do I cut this back or just let it stay as it is in my garden?

Doug says that allium flowers are much like any other flower. If you like the look of the seedpod (and many folks do) then you let them sit there to enjoy the starlike look of them. If you don't like that look, then you prune them as far down the stem as you can to remove the stem. Leave the leaves alone to grow and replenish the bulbs for next year.