If you don't want Classic to ever start up, Brian M. sent in an easy solution - simply move (in case you want it later) the file /System/Library/CoreServices/Classic Startup.app to a new location. From the terminal, you'd just do
mv '/System/Library/CoreServices/Classic Startup.app'[space][Replace [space] with an actual space character, and enter as one command].
'/path/to/new/location/Classic Startup.app'
You should be able to do this by removing "Classic Support" and "Classic Support UI" from the 9.1 System Folder. You'll get asked when Classic is about to start whether you want to install certain items that Classic needs; I think if you answer "no", then Classic won't start up.Either of these methods should prevent Classic from launching.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010422094944759