Hello there!
Cyber-stalking is now more common than physical harassment. Cyber-stalkers usually have a disturbed obsession with their target, sending them unwelcome messages or emotionally harassing them, like cyberbullying, except more deep-rooted and with generally worse intentions, wanting their target to fall into some sort of submission. They commonly have four objectives:
1. locate
2. Surveil
3. Emotionally harass
4. Criminally manipulate their pray.
There are various means by which you can assertively and legally protect yourself from these people. Starting with low-key assertive e-mails perhaps is the best way to begin. If it escalates, a good thing to do would be to call the law enforcement.