First, if you can, use the export GUI to print to grayscale. In the figure window, select File>Export Setup to open the GUI. Under properties, select Rendering. In the Colorspace drop down menu, you can select grayscale.
Second, you can print from command line. The method you chose depends on the kind of image you are trying to print.
If you are trying to print an image with an associated colormap you can convert your image to grayscale first before printing to eps. For instance:
surf(peaks);
colormap(gray);
print -deps testimg
If you are trying to print a plot, with which there is no associated colormap, you may have to use HGEXPORT:
plot(rand(10))
style = hgexport('factorystyle');
style.Color = 'gray';
hgexport(gcf,'test.eps',style);
