Just nip the seed head off, and as Spudly says; If the bulbs are big enough use these up now
However I only partly agree with his second statement; If left to late, the onion will have a woody stalk through the middle and become no good.
I wouldn't say they will be no good, true they may have an inedble centre core but the flesh around this core will be perfectly edible.
The only thing I would add is eat these onions first as they wont store as well!