Hi Durga,
Currently pop-up is mandatory when user clicks logoff.
The scenario Etay referenced is when a child window of portal is being opened and then closed using the (x) button you can omit the popup using that note.
The feature to disable the popup for logoff is currently under Proof Of Concept as it is a very delicate place in the portal and regressions there are security issues since sessions will remain open.
Hopefully it will be available soon.
BR,
Saar