Nouveau seems to be both difficult to disable and difficult to spell.I had a little bit of trouble installing official NVIDIA drivers off of the NVIDIA website in my fresh install of Ubuntu 15.10 however, after figuring it out it was actually quite a simple process.
This is a tutorial for if you want to install the latest drivers that NVIDIA offers on the website, otherwise you should just go into "Software & Updates", navigate to the "Additional Drivers" tab, and select the NVIDIA proprietary binary that you would like to use.
Here are the links to NVIDIAs drivers:
NVIDIA drivers: nvidia.com/Download/index.aspx
NVIDIA beta drivers: nvidia.com/Download/Find.aspx
For the purpose of this tutorial the filename of my binary is:
NVIDIA-Linux-x86_64-358.09.bin
And it will be saved in my Downloads folder in my home directory, which will be represented as either of the following:
~/user/Downloads/
/home/user/Downloads/