No one other than someone who has an administrative account or knows an
administrative account username/password can log you off without
resetting/powering down the machine. If everyone is an administrator on the
machine - then anyone can log you off.
When they start to log you off, they are told that "UserX has an active
logon, if you continue, this will log UserX off and they may lose work" or
something along those lines - otherwise - any administrative account can log
you off if they choose to ignore (or pay attention to and maliciously log
you off) this warning.