What happened to the spirits of the dead giants?

The spirits of the dead giants became evil spirits that dwell on the earth, where they continue to afflict, oppress, and destroy humanity without facing judgment until the final consummation of the age.

These beings arose because the giants were born from the union of the holy Watchers and human women, so that evil spirits proceeded from their bodies and were appointed to remain on the earth rather than in heaven (1 Enoch 15:6-10). They take no food yet still hunger and thirst, rising up against the children of men and women from whom they originally came, working destruction, battle, and offences across the world (1 Enoch 15:11-12).

Spirits from the flesh of the slaughtered giants go forth to destroy freely until the great judgment over the Watchers and the godless brings the present age to its close (1 Enoch 16). At that final judgment the demons released from the giants' destroyed bodies will be permanently confined, ending their long influence over humanity, though for now they remain active as a partial fulfillment of the earlier binding of the Watchers themselves (Prophecies: Binding of Evil Spirits).