All your settings are saved inside a file called
config.cfg.
You can access this file by getting inside of your hidden
~/.nexuiz-folder by opening Finder, selecting
Go to ->
Go to folder... and typing
~/.nexuiz
inside the text field.
A new Finder window will open, and you'll see the ~/.nexuiz-folder with a subfolder
data.
Inside that folder is your
config.cfg. Delete this file and the next time Nexuiz will start up with default settings.
The reason why redownloading and reinstalling doesn't work is that this hidden Nexuiz-folder is not being deleted with the game application. This is quite useful if you get a new version of the game and don't want to reconfigure all your precious settings after the update.
This hidden folder also stores your download cache and your recorded demos and screenshots.