Dear John Email?

There is a guy I recently had a date with. We didn't really hit it off. I've tried to tell him I'm really busy with work, can't talk, can't go out, etc. He just doesn't seem to get that I'm not interested in seeing him again. The question of the day: How does everyone feel about a "Dear John" email? I don't feel like I owe him a phone call explaining that I'm not interested, since I haven't known him long at all, he's been inappropriately forward, and I just don't want to. So I suppose the question I have is this - should I send him an email or just continue to ignore his calls?