ASP/Exchange Server Problem

Hello All!

I have an asp web page(seems to be generated from somewhere maybe OWA) when we click a Login Button. it open another Browser window to connect to Exchange Server, then it prompts for the username n password.. when the user logs off.. that browser window is closed.
Now if the user clicks on the Login button again. he's not asked for username/password... i read OWA documentation n it says that u have to close ALL Browser windows... okay.. i can do this.. but what to do about the users spread in all our the country?
My Question is.. is there any way to expire the user's session with Exchange Server so that everytime he clicks the Login button the username/password prompt appears... ?
or
is there a way to clear the user's credentials stored by browser without closing it?