You could try uninstalling the driver directly, preferably purging it:
Code:
aptitude purge xserver-xorg-video-nouveau
You could also try to blacklist the module, but for some reason, the last time I tried adding
blacklist nouveau to /etc/modprobe.d/blacklist.conf, it didn't work. I read on the net a similar case of someone not being able to blacklist it by the usual means, but now I can't remember what was the solution. Anyway, what I did was purging the nouveau driver package; after that, I could install the proprietary driver.
Regards.