Plant them out in the garden and let the leaves die back naturally. They will flower again next year but not early and not as well... eventually they will revert to Spanish bluebells.

Remember to harden them off first as you would for any plant going from in to out.

Moongrower wrote
"eventually they will revert to Spanish bluebells."

Are you sure about that Moongrower??
English bluebells are Hyacinthoides nonscripta
Spanish bluebells are Hyacinthoides hispanica
Hyacinths are Hyacinthus orientalis

Sorry, but I have to disagree with you.

I find that if you put them in the garden and feed them as other bulbs they keep going pretty well.

They never really have the huge flowers that they have in their first year, so we can forgive Moongrower for thinking that they were morphing into another species!!

I have seen this "reverts to ordinary bluebells" in a few places. Odd belief in a way. We have been planting the old forced Hyacinths in this garden for the last 15 years and they are all still coming up , flowering now, looking as good as ever and STILL completely Hyacinths not Bluebells.
Plant them out as soon as possible and give them a feed. Do not damage the leaves as they need them to feed the bulb for next years flower.

We found that they went so far back if they weren't Spanish bluebells that at least is what they looked like -realistically no, of course they can't change species :-)

I always plant my Hyacinths straight into the garden and have a large collection now, they come up every year, very faithfull and are beautiful.

I have the same experience as Grandmage.

Just added a picture of last years forced ones flowering in the garden today.