On discussing the idea of eruv techumin, the gemara says (7 lines from the bottom of amud bet) Ba'u Mineih Mi'Rabah - "they asked Rabbah about a person who was taken out of the techum Shabbos by force, and who cannot move more than 4 amos. That person now must relieve himself, and we know that he must find a "makom tzanu'a", a modest place. If there is no modest place within 4 amos of his place, can he move more than 4 amos to relieve himself?" Rabbah answered "yes", and adds, "so great is the idea of Kavod ha'Berios, that is even is "doche", pushes aside a "lo sa'aseh" of the Torah!" Since Techum Shabbos is a gezeira d'rabanan, Rashi says that the "lo ta'aseh d'oreita" is "lo sasur", don't turn aside from the gezeiros of the chachamim. This seems to clearly imply that (at least according to Rashi) every "lav" of the Rabanan is also a "lo sa'aseh min haTorah". Is this correct? Aren't there Rishonim that don't hold of the idea that every "lav d'Rabanan is also a "lo sa'aseh d'oreita"? And of those Rishonim that hold otherwise, what do they do with this gemara?
(a) That assumption is correct.
(b) Good question. We could also ask from the Gemara in Berachos (19b) and from other Sugyos in Shas which mention "overriding a Lo Sa'aseh d'Oraisa" with regard to an Isur d'Rabanan.
The RAMBAM's opinion (Sefer ha'Mitzvos) is that every Mitzvah d'Rabanan also includes the Isur d'Oraisa of "Lo Sasur." The RAMBAN (Shoresh Rishon), though, argues and says that the Isur d'Oraisa of "Lo Sasur" does not apply to Mitzvos d'Rabanan. He explains that when he Gemara uses the term of Isur d'Oraisa with regard to Isurei d'Rabanan, it means it in the sense of an "Asmachta," and not that it is actually an Isur d'Oraisa.