What would make a guy keep harassing me when I told him to stop?

I told him I don’t want him in many ways, politely and very mean. I even threatened to call cops on him. However he still reaches out to me by constantly calling back to back blocked and popping up to my house unexpectedly. This is someone I don’t want at all and I’m tired of him and want him out of my life altogether. He’ll stop calling for a few days- weeks then starts his bullshit again. He’s 24 years old so he’s younger than me, not sure if that matters.

He’s mentally ill
He’s desperate and pathetic
He’s obsessed with me (potentially dangerous stalker)
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What would make a guy keep harassing me when I told him to stop?
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