Because the PIXMA MX870 uses individual ink tanks, the option to turn off each ink tank is not available.
If you wish to only print from the PGBK ink tank, please set the printer driver to Plain Paper and ensure that the item that is being printed is black. The PIXMA MX870 will combine all of the color inks to make gray if the original text is a color other than true black.
Thanks for the quick answer. Ok, the driver (in printer properties) is already set to "Plain Paper". But how exactly, do I ensure that the doc to be printed is only b&w? Many docs have a combination of b&w & color, and when I print, I select "Black & White" or "Print in greyscale" in the print properties; is that enough to tell the printer to use only the b&w ink and not waste color ink on b&w prints?
Side note: I've noticed that when printing from Chrome, even if I set the Color setting to "Black & White", it still prints color items in color. I have to go into "System print dialog" (using Ctrl+Shift+P), and select "Grayscale Printing" in that dialog, and only then will it print everything in b&w. (If "Grayscale Printing" is set from the windows printer properties dialog, it will stay checked as default for future prints).