Probably the easiest way to get them to leave you alone is to ignore emails and phone messages like that one. People feed off response. Don't give them any fuel and they will die out. It may take 30-90 days, but they will eventually die out. In fact, you will be amazed at how quickly they really do forget all about you and that will be another time of vulnerability that might tempt you back.
Because everyone needs to feel special and important and have a sense of belonging to some sort of social group. Find something else to replace it otherwise, nature abhors a vacuum and you might be sucked back in.