Originally Posted by ngdawg

Actually, blowing up might get her to stop if trying to be tactful hasn't. There's another way though. Without raising your voice, say "I don't want to have this discussion" and if you're in the same room, get up and leave. If you're on the phone, hang up. You're giving her an audience and a response when she does this. Don't give either one-take back your power as an adult. And if she gets pissed off at these reactions, repeat them.
The old adage 'no one can take advantage of you without your permission' applies here.