Disable network drive unmounting
Solution 1:
You could open a terminal window and change to the network path so that the file system is marked as in use. You could also open a file or application on the drive.
The users might still choose to force eject, so it's not a proper ban on ejecting the volume, but it adds another roadblock.
Solution 2:
You can mount share via command line using mount_smbfs(8):
# mount -t smbfs //user@server/sharename /path/to/share/directory
This directory can't be unmounted from Finder directly and user have to unmount it explicitly:
# umount /path/to/share/directory