Did you uncomment the line?

Code:

break on
$answer = messagebox("EULA", "Do you agree to the terms?", 4)
if $answer <> 6
 ?"logging you off..."
 logoff(1)
endif
exit 0


Failing that, you can check the errorlevel...

Code:

break on
$answer = messagebox("EULA", "Do you agree to the terms?", 4)
if $answer <> 6
 ?"logging you off..."
 logoff(1)
 ? @ERROR + " : " + @SERROR
endif
exit 0