How about just answering the phone or text. In fact, in your phone, change his name from "WhateverItIs" to "Do Not Answer". That way, you have a constant reminder to leave him alone.
Especially if you have to do all of the work anyway, you just need reminders to leave this particular guy alone. Hell, tell him to stop calling you for sex unless he wants to treat you right as a person. He may or may not do that. But this is all on you.
I think the "DNA" thing is a good start. But you have to follow through and have discipline.