Also, I think it's important to remember that this woman is, at least in the OP's opinion, a light acquaintance. If this were a long-term friendship of some depth, having a heart-to-heart conversation or being blunt might be good ideas, but to me at least, they seem unnecessarily dramatic and involved for someone the OP really doesn't know that well. (At least, that is how I interpret the relationship; please correct me if I'm wrong.) I think the OP is not trying to give her the cut direct, but rather is fine with seeing her, say, once a year at the college reunion.
What about something that includes, "We don't have time for new friends at the moment"? It seems less blunt than "We don't have anything in common."
Although I agree that some language involves "stringing people along" and that's rude (like promising to call at some point when you know you won't), I personally feel the "busy/no time" route is a socially acceptable way to disengage from a friendship. It's true that some people don't pick up on its true meaning; and if they don't, one may have to try other techniques. This woman also doesn't seem to pick up on the fact that one shouldn't unload all one's problems on a near-stranger in long rants with repeated emails and phone calls. So, if "busy/no time" doesn't work, the OP may indeed have to go with "nothing in common" eventually, but personally I would start with the softer approach.