Hi Walter,
If you are deleting the session, when the user is on the login screen, then obviously you will not be able to login. The login screen, like any form, relies on session values to work - if you delete the session half-way though, you're gonna make it fail.
Perhaps you should reconsider your need to "delete the session" in the first place. Perhaps what you should be doing is clearing specific session values which you feel should not exist between users.
If you wish to delete the session you will
a) need to remove the implicit "logout" usually in the Login Form and
b) provide an alternate logout to the user.
Of course when a user changes from one to another, _without_ an intermediate logout, then you may also need to do extra management there.
I think simply re-assigning the session values that you feel are an issue is really what you want to do here- not delete the session.
cheers
Bruce