screen locking

Guys, I work in a college enviroment where students occasionaly enter my room when im not in to see other staff. This means i need to be able to properly lock my machine using the "lock the desktop" icon to stop others using it. But on the new suse 8.1, it has a button "start new session", which if a student pressed would allow them to re-logon using their passwords. This would log me out, which is annoying rather than destructive. While i would easily be able to discover who did this, id rather just not have that button. is there an easy way of disabling it? cheers, Elf.