CAN DEMONS REPENT AFTER THEY SINNED?

It is not possible for demons to repent after t heir sin.  According to St. Thomas, the will of an angel cannot change after it has made a choice because the angel (or fallen angel) sees with much greater clarity the object of his choice and hence his decision about that thing is irrevocable.  Angels are mid-way between the immutable God, whose Mind and Will are unchangeable, and human beings, whose intellext and will are clouded by the flesh and therefore always changeable.  Humans have to be trained to choose the right things over time but are still free to choose evil.  Angels, however, don't have to be trained.  Once they grasp something, they know it in its entirety and they cannot change their choice for good or for evil once they have chosen it.  That is to say that their choice of evil is always a mortal sin because they always give a deliberate consent of the will with full knowledge about matters which are always grave, and because of their superior knowledge, they remain for all eternity ostinate in their sin.

By Fr. Thomas J. Euteneuer, "Exorcism and The church Militant" page 33.