First of all, I strongly recommend you to try a slightly more recent version of Slackware than 9.1. 11.0 is just around the corner, and in the meantime, 10.2 is rock solid. A lot of changes have gone into Slackware since 9.1, and it's quite likely that the issue you are describing is fixed there.
As for your shutdown issue, try creating a file called /etc/shutdown.allow
and update the inittab file to ensure that the shutdown program is called with "-a" argument.
shutdown.allow has one username per line, and shutdown compares this list to the list of locally active tty's to prevent remote user shutdowns.
Note - haven't tested this myself, so you might want to do a couple of dry-runs before letting your users have a go