When you use the words "won't leave someone alone" it makes me think of a stalker. If someone is obsessing on someone and that someone wants them to leave them alone, then there's a definite problem.
I agree with Clough, though easier said than done. The reason for the obsession needs to be looked into. Like, is this a common thing for this person, to be obsessive. Do they have an addictive personality? Obsession is not love, especially when the person wants to be left alone. It's definitely not healthy. People do not obsess over people they love. Although it's natural to care about their well being when you love someone, it's not normal to only think of that person all the time. Something is lacking in their own life for sure and self-help books or even a therapist might help.