[cups.general] Howto force a printer to print Grayscale?

[Linuxhippy]

Hi there!

I own a Canon-i250 (I know about Canon :( ) and the manufacturer 
provides drivers for cups.
They work great except that I want my printer to print black only (so 
that greyscales aren't mixed out of Colours), but I don't know where to 
change this.
The kprinter-dialog tells me that the only supported operating mode is 
"RGB", however I believe (I am not exactly sure) that it was possible 
with an older version of cups (shipped with SuSE-9.1) to print true 
grayscale too.

Is there any way to force the printer to print black only?

Thank you in advance, lg Clemens