A tad more than just a bit dated, but this article explains one means:
http://www.pcworld.idg.com.au/index.php/id;166575994;fp;2;fpid;1976181544
The good news is that you can completely remove Mac OS X reasonably painlessly, without disrupting your OS 9.1 setup. It just takes time, and it's a little fiddly. You'll also need to use ResEdit, but don't worry - it's not really a hack.
This link
http://www.macandstuff.com/weblog/index.php?/weblog/removing-mac-os-x/ suggests the following pretty scenerio
Removing Mac OS X? Why would anybody want to do this? Dont do it if you are not sure what you are doing. Mac OS X is a much more sophisticated and stable operating system than the OS 9. Oh, well, if you really want to here is the brief (and barbaric") procedure. Enjoy OS 9.
The general process of uninstalling Mac OS X without erasing the volume is:
1. start OS 9
2. move to Trash: non-OS 9 Applications folder; System (not System folder"); Library; Users; and if visible: mach, mach_kernel, and mach.sym
3. find a utility that can locate and delete invisible files from the root level of the hard drive: .hiden, dev, usr, Network, etc, var, Volumes, private, .vol, bin, sbin, .DS_Store, cores, tmp, .Trashes
4. hold down the command key and options key at startup in OS 9 to rebuild the desktop
Important Note: If you are not sure about what all of this is, then DO NOT do it.
