It seems to me that this is one of those occasions where complete silence is a proper response. After you've thanked her once, the next time she brings it up, just smile and say nothing. And I mean, say absolutely nothing. Sooner or later, she's going to feel the need to break the uncomfortable silence. She will either further comment on whatever she wants you to thank her for, in which case, you maintain both the smile and the silence, or she will eventually change the subject. Do this enough times and you will train her not to fish for further thank yous as she will learn that it will get her nowhere.
By the way, this is not an easy thing to do as it is very tempting to be the one to break the silence. But if you can pull it off, it should work eventually.