Here are some alternative phrases, depending on the context:
If you are talking about the act of following someone obsessively:
* Harassing
* Obsessively pursuing
* Following
* Surveilling
* Spying on
If you are talking about someone's behavior in general:
* Obsessive
* Fixated
* Unhealthy attachment
* Controlling
* Threatening
If you are writing fiction:
* Shadowing
* Hunting
* Preying on
Remember: It is important to be clear and direct when discussing stalking. If you or someone you know is experiencing stalking, it is essential to seek help and support.