This went back and forth four or five times until I then reminded him that I had made it very clear that I was not available, and that he was losing my time and

This went back and forth four or five times until I then reminded him that I had made it very clear that I was not available, and that he was losing my time and his own by sending these constant emails. The real problem is that you kept continuing the conversation, even though you said no. That probably gave the individual some sort of hope that they could convince you otherwise. I let him know that I had set up a filter to route his email address to trash and that he would never hear about me again. I personally would recommend against doing things like this. Don't tell them you're going to do it, just set and forget. ;)