Force any printer to convert all colors to Grayscale automatically?
A "one-size-fits-all" approach may not be possible, as different printers have different capabilities and driver controls for specifying Black ink only. 

You could try the Color Matching pane of the print dialog: select ColorSync, and select a Greyscale or Black &amp; White color profile. However, even if you do this, some printers may still try to print this as a '4-color Black', to spread the ink usage across all the cartridges.

Your best option is to determine what controls your printer has (if any) for printing to Black ink only: then select them in the print dialog and save as a Print Preset.