sugardaddy, I know you likely are looking for more concrete information on how to solve this problem for your girlfriend but honestly, this sounds like something SHE needs to handle herself. If she is not interested in this person, it is a simple matter for her to tell him, "I am not interested in you, please stop calling me, approaching me and talking to me." She IS a grown woman, isn't she? If she is a teenager, perhaps I can understand she might need some advice on how to handle this (such as she has sought from her minister) but still, she needs to handle this herself. If she feels she is in danger, she can call 911/the police.
I advise you to take a big step backwards, out of this situation. Have you gone to Al-Anon?