How to get him to leave me alone?

There's a guy who's really crushing hard on me, but I'm just not interested. I've told him a million times to please stop, but I don't want to be mean because he really is a nice guy. He's been obsessing over me, sending me emails and calling me and...

everything, but I really just want it to stop. How do I get him to leave me alone?

just ask him in one of the convos if he likes you then be like if he says yeah he likes you just say aw too bad I only see you as a friend or that you like another guy or constantly talk to him about a mystery guy that you like either you can go with the truth and say I dont like you or lie your a** off your choice my words

I know from experience that guys like that are NOT nice. and If they can't take no for an answer, you said it yourself, he's obsessing. That's not good.
Go ahead and be rude. Being nice isn't working. Ignore him and don't reply to his emails. Hopefully he will go on to someone else. People like this can be dangerous.