They've obviously got some issues that require professional help. This doesn't seem that far off from stalkers who feel like they are part of a person's life, and turn nasty when rejected (even though they never were a part of the person's life).
That's exactly what they are, only this is the online variety. The hardest problem of all for me in dealing with this is that you simply can't reason with these people.
The only thing you can do is ignore them. Become a stone wall. If the stalker can't seem to get any reaction out of you, he/she will eventually get bored and start stalking someone else instead.
In the absence of a reaction/interaction mentally disturbed people will make up their own in their head (i.e. he really wants to talk to me, but he's busy; sure he's married, but she holds sway over him and it's obviously a loveless marriage; etc). There is no logic to be applied here.