are the two on the same partition?
If not the answer is easy, simply reformat the partition with OS X on it. If both systems are on the same partition, you have a problem. As far as i know, there is no actually uninstall procedure. The best i can recommend is to delete files that you know are part of OS X
Applications, but not Applications (Mac OS 9) System, , not "system folder"
Documents, Make sure you take out things you need first!
The Mach Kernel and other misc. files.
That should eliminate the bulk of OS X's files.
OS X will also no longer show up in the startup disk panel.
That said, and if everything is on one partition, why not boot into 9.1 and delete the OS X System folder? This won't get rid of all the X stuff but it's a start. You can continue to backup and delete files a few at a time to see if it's OK to trash 'em.
Or, Copy all the 9.1 control panels, extensions, prefs, etc to a backup disk, reformat the drive and re-install a clean 9.1. Then add all your old files back.
However! I believe OS X does something to your 9.1 system. I'd check with people more knowledgeable than I am before doing this...