We understand your distress and commend you for repenting and turning back to Christ. It is only in Him that we find true freedom and forgiveness. Remember, "If we confess our sins, he is faithful and righteous to forgive us the sins, and to cleanse us from all unrighteousness". (1 John 1:9 WEB).
Firstly we should address the situation and identify the sin. The situation was adultery this is very clear. "Let marriage be held in honor among all, and let the marriage bed be undefiled, for God will judge the sexually immoral and adulterous". (Hebrews 13:4 WEB). We know that you have repented and we are not condemning you for this sin. It is clear that this person is also sexually immoral and preys on vulnerable people. We will keep this person in prayer as well.
You are right to seek the removal of this person from your life. We must rebuke you for not seeking this earlier as harassment and stalking can lead to more serious crimes. The fact that you saw a darkness in this person should have been your first warning. We are also to test the spirits to see if they are from God. We know that in the end days there will be false prophets, false teachers, false believers and even false Christs. "Beloved, do not believe every spirit, but test the spirits to see whether they are from God." (1 John 4:1 WEB).
We will pray for your protection and for this person to be removed from your life. We also pray that this person seeks Christ and repents. We know that "The thief only comes to steal, kill, and destroy. I came that they may have life, and may have it abundantly." (John 10:10 WEB). Jesus came to give life and only good comes from Him. In your request you did not mention Jesus or even hint at His name. It is only by the name of Jesus that we have access to God. "Jesus said to him, 'I am the way, the truth, and the life. No one comes to the Father except through me.'" (John 14:6 WEB). We hope that you will come to Jesus and submit your request to Him as well as all other requests in the future.
Dear Lord, we come before You, lifting up our brother/sister in Christ who is being harassed and stalked by someone from their past. We rebuke the spirit of darkness that works through this person and command it to leave in the mighty name of Jesus. We ask You, Lord, to intervene and remove this person from our brother/sister's life. We are praying for a Christian man or woman to be their spouse to keep them from this temptation of adultery in the future. Protect them and guide them in Your will. We also pray for the person who is doing the harassing, that they may come to know You, repent, and be saved. In Jesus' name we pray. Amen.
Going forward, trust in the Lord and seek His guidance. Psalm 91 is a great Psalm of protection to pray and believe. Stay strong in your faith and walk with the Lord. He will guide you and protect you.