Senhores, descobri como barrar para os usuários o botão desligar, no debian 6 entrar em /etc/gdm3 e modificar o daemon.conf
# GDM configuration storage
#
# See /usr/share/gdm/gdm.schemas for a list of available options.
# Some settings can also be configured using the gdmsetup tool.
[daemon]
AutomaticLoginEnable=false
AutomaticLogin=
TimedLoginEnable=false
TimedLogin=
TimedLoginDelay=30
Greeter=/usr/lib/gdm3/gdmgreeter
RemoteGreeter=/usr/lib/gdm3/gdmlogin
DefaultPath=/usr/local/bin:/usr/local/sbin:/sbin:/usr/sbin:/bin:/usr/bin:/usr/bin/X11:/usr/games
RootPath=/usr/local/bin:/usr/local/sbin:/sbin:/usr/sbin:/bin:/usr/bin:/usr/bin/X11:/usr/games
User=gdm
Group=Debian-gdm
LogDir=/var/log/gdm
PidFile=/var/run/gdm.pid
PostLoginScriptDir=/etc/gdm3/PostLogin/
PreSessionScriptDir=/etc/gdm3/PreSession/
PostSessionScriptDir=/etc/gdm/3PostSession/
DisplayInitDir=/etc/gdm3/Init
XKeepsCrashing=/etc/gdm/3XKeepsCrashing
RebootCommand=/sbin/shutdown -r now "Rebooted from gdm menu."
HaltCommand=/sbin/shutdown -h now "Halted from gdm menu."
SuspendCommand=/usr/sbin/pmi action sleep
HibernateCommand=/usr/sbin/pmi action hibernate
ServAuthDir=/var/lib/gdm3
BaseXsession=/etc/gdm3/Xsession
SessionDesktopDir=/etc/X11/sessions/:/etc/dm/Sessions/:/usr/share/gdm/BuiltInSessions/:/usr/share/xsessions/
DefaultSession=default.desktop
UserAuthDir=
UserAuthFBDir=/tmp
UserAuthFile=.Xauthority
StandardXServer=/usr/X11R6/bin/X
Xnest=/usr/X11R6/bin/Xnest -br -audit 0 -name Xnest
FirstVT=7
VTAllocation=true
SoundProgram=/usr/lib/gdm3
############################################
[security]
AllowRoot=false
AllowRemoteRoot=false
AllowRemoteAutoLogin=false
RelaxPermissions=0
CheckDirOwner=true
DisallowTCP=true
############################################
[xdmcp]
Enable=true
Port=177
###########################################
[greeter]
ConfigAvailable=false
Browser=false
MinimalUID=1000
Exclude=bin,daemon,adm,lp,sync,shutdown,halt,mail,news,uucp,operator,nobody,gdm,postgres,pvm,rpm
IncludeAll=true
SystemMenu=true
SecureSystemMenu=false
###########################################
[chooser]
HostImageDir=/usr/share/hosts/
Hosts=
Broadcast=true
Multicast=false
#########################################
[debug]
Enable=false