The Rosh שבת פרק ב' סימן ט"ו asks, "How can the Geonim make a Gezeira after the sealing of the Talmud?"
The Kesef Mishneh (Mamrim 2:1) writes that just as we find throughout the Talmud, that Amoraim cannot argue on Tannoim, because at the time of the sealing of the Mishneh, we accepted the Mishneh's authority. When, so too when the Talmud was sealed, we accepted its authority, and cannot argue on it.