We understand that you're going through a troubling situation where you feel overwhelmed by thoughts of your former manager. First, we want to remind you that God tells us to take every thought captive to obey Christ (2 Corinthians 10:5, WEB). This means we have the authority, through Jesus, to control our thoughts and not let them control us.
However, we need to address something before we proceed. We noticed that your request does not mention Jesus, Jesus Christ or His name. It is only by His name that we can be saved and "For there is no other name under heaven given among men by which we must be saved" (Acts 4:12, WEB). By the name of Jesus, in His name, and to His name; the Devil and demons will be defeated and flee from you.
Regarding your former manager, it's clear that you're feeling invaded and even obsessed, and it is frustrating you. We need to remember that God calls us to love our enemies and pray for those who persecute us (Matthew 5:44, WEB). This doesn't mean we excuse their behavior, but rather, we trust God to deal with them justly.
However we also understand it is normal to have feelings of anger towards people that may have wronged you in the past. It is unfortunate the prayer request is not clear whether the person has been just treated harshly emotionally or is actually experiencing spiritual warfare. If the former manager is actually tormenting you spiritually; we must understand that in Roman's 8:31 that if God is for us then who can be against us? God is sovereign and will protect you in both physical and spiritual areas. Understand that the Devil "prowls around like a roaring lion, seeking someone to devour (1 Peter 5:8, WEB). We urge you to put your full faith and trust in Jesus and the scriptures to fight against Satan. The scriptures are clear that spiritual warfare should be addressed with scripture. "Stand therefore, having the utility belt of truth buckled around your waist, and having put on the breastplate of righteousness" (Ephesians 6:14, WEB). Spend time daily in the scriptures and Jesus' power can free you from thoughts from the Devil. Finally, we must make it clear that the behaviors your manager is demonstrating do no align with God's word. "Be self-controlled and sober-minded. Be watchful. Your adversary the devil prowls around like a roaring lion, seeking someone to devour." (1 Peter 5:8, WEB).
If your manager is making unwanted romantic advances and is also obsessive then this must be addressed. It is clear that these are inappropriate and must be stopped. If you have a human resources department then report it; if you are on your own prayerfully ask God if you should take other action and screen your manager's calls and emails. Do not entertain their irrational calls. Furthermore, it is clear your manager's behavior is not reflective of God's direction for our lives. You are not to be bound with this person.
We want to encourage you to pray for this man, that God would change his heart and bring him to repentance. Remember, vengeance is God's, and He will repay (Romans 12:19, WEB).
Let's pray together:
Father, in the name of Jesus, we come before you to lift up our fellow brother/sister in Christ. We ask that you would help them to take every thought captive to obey Christ. Lord, we rebuke any obsession, fear, or thoughts that are not from you, God, and cast them away. Help them to forgive and let go of mistakes made in the past and those around them that may be disturbing them, and to trust you to deal justly with those who have wronged them, Jesus. Protect them physically and spiritually from your adversary, God. If they have not done so already, help them to put their full faith and trust in Jesus and His name. Please bring their manager to repentance, God, and change their heart. We ask all in Jesus name because God did not give us a spirit of fear, God, but of power and love and self control. You have ultimately defeated the devil. (2 Timothy 1:7) Help us to know this power victoriously in our lives.
Please consider spending dedicated time in the prayer and scriptures daily to find peace.