If there is physical access to the server, there is no good protection from booting server off CDROM or floppy (or simply stealing the server). The only (presumable) good option is to encrypt file system on the server. This is not (yet) supported by SME.
You can chain server to the rack or desk, you can disable cdrom and floppy drive, disconnect and disable keyboard, but usually why not just limit the access to it by locking it in the (server) room?