Linuxers posted a guide about removing the Nouveau drivers from Ubuntu

Before starting, you might want to know that nouveau is a project which aims to develop open source drivers for Nvidia cards. When you install Ubuntu, it detects your card and installs these drivers.
Please note that, nouveau drivers manual removal is required only if you are going to install the propreitary nvidia drivers yourself. If this is not the case then directly install the required graphic drivers from System->Administration->Hardware drivers. Its the recommended and the most convenient way available.

  How to remove or uninstall Nouveau drivers from Ubuntu