session

Hi,
I have a website which requires the user to log in. after log in they will then be redirected.
I have use the following code to create a session
If UserExist Then

            Session("user") = txtUser.Text

            Response.Redirect("Admin.aspx?Parameter=" & Server.UrlEncode(txtUser.Text))

        End If
Then in the redirected page I have the following set of code which grabs the username.
If Session("user") IsNot Nothing Then
            lblUsername.Text = CType(Session("user"), String) + ","
        End If
The problem is how do I stop the session after the user logs out. by logging out the user gets redirected t ...