Dear Ex Lover,
"Perhaps you didn't understand the last time I told you to stop contacting me, so I'll do my best to spell it out for you. I do not wish to have you in my life anymore.
I don't know how much more clear I can be about it. It would serve you best to move on with your life and find someone who can put up with you, because I'm done trying.
"I hope this is enough closure for you.
"P.S. If you need me, you know how to find me."