You mean you do not want user to close the pop up window. I think it's impossible, and it should be impossible in the stand of user. The user should have the right to close the window that he does not want to see.
I'm not sure what you want the popup for... but have you considered using divs to mimic a popup window? See http://www.astral-consultancy.co.uk/cgi-bin/hunbug/doco.cgi?11540 Hope this helps