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.