I used all kind of CTRL/ALT/Shift mapping and I saw sometimes they activated actionbars on my alts.

The only workaround I found was to map the "unused on this toon" key combination to an action bar slot in bartender (which has like 50 keys mapped to it

Mapping in in the bindings-cache to none does only work for predefined keys. (as far as i can see)

cheers