The non-breaking space is a variant of the space character which as the name suggests prevents automatic line breaking when using a space character. Another common use of it is to avoid collapsing of white-spaces in formats such as TeX or HTML. There are also some others specific typographic uses but nevermind.
setxkbmap -option "nbsp:none"
However this won’t work as you might expect it especially when you have multiple keyboards with different layouts. So another solution would be to specify XkbOption directly within the Xorg configuration file, one for each input device. But this won’t work neither if you use a Bépo layout as this option will simply disable the use of underscore (AltGr Space). So the final solution is based on xmodmap to modify the list of keysyms assigned to the space keycode (0x41). You may just add the following line to ~/.xmodmap and ensure that the file is loaded when your session starts with xmodmap ~/.xmodmap.
keycode 65 = space space space space underscore underscore space space