So a fallen angel can't stop being a fallen angel.
If you don't mind me asking--and I'm only asking to foster a bit of deep thinking here--why?
The Bible does not specifically address the issue of God's giving the fallen angels an opportunity to repent, but we do know principles and can extrapolate from them an educated guess. First, Satan (Lucifer) was one of the highest angels, perhaps the highest.
Ezekiel 28
14 "You were the anointed cherub who covers, And I placed you there. You were on the holy mountain of God; You walked in the midst of the stones of fire."
Lucifer and all the other rebelling angels were continually in God’s presence and had a complete knowledge of the glory of God, unobstructed by the things that keep mankind from knowing Him fully. Therefore, they had no excuse for rebelling against God and turning away from Him. For Lucifer and the other angels to rebel despite what they knew about God is the worst sort of evil, and resulted in God's decision to not give Satan and the other fallen angels the opportunity to repent.
Second, God did not provide a plan of redemption for the angels as He did for mankind. The fall of the human race necessitated an atoning sacrifice for sin, and God provided that sacrifice in Jesus Christ. In that plan of redemption, God redeemed the human race and brought glory to Himself for His grace, love and mercy.
No such sacrifice was planned for the angels. In addition, God referred to those angels who remain faithful to Him as His “elect angels."
1 Timothy 5
21 I solemnly charge you in the presence of God and of Christ Jesus and of His chosen angels, to maintain these principles without bias, doing nothing in a spirit of partiality.
We know from the biblical doctrine of election that those whom God elects to salvation will be saved, and nothing can separate them from God’s love.
Romans 8
38 For I am convinced that neither death, nor life, nor angels, nor principalities, nor things present, nor things to come, nor powers,
39 nor height, nor depth, nor any other created thing, will be able to separate us from the love of God, which is in Christ Jesus our Lord.
Clearly, those angels who rebelled were not “elect angels” of God.