Very problematical as this guy is stalking you.
If you have not responded, don't indicate in any way you received the letter. Beware of certified mail you're not expecting.
The first defense to defamation is that the defamation is true. You can defame someone, provided you can prove all of it. This info you got from others may or may not be true. You have no way of actually verifying this. He may be posing as some of those people. It would be prudent to take the blog down now. If anything you have posted is untrue, he may have you.
If he attempts to contact you, do not respond. Call the cops. Keep records of everything: what you already have and dates, times when he calls, letters you receive, texts or emails.
Block all of his known addresses. He can change email addresses, so if you're really serious, change yours. Go through all of your online profiles and set them to private. Remove all personal information. Post you moved on your facebook prior to setting it to private. Google yourself, look at the listings: have your address/phone etc removed from anything you didn't ask for it to be on. This may take some work. I am fastidious about my privacy, but can find my info all over the internet anyway. A lot of info is public record so an intrepid stalker can find you.