Delete Cookies and Sessions when page loads

Hi,
I have a Page "Main.asp" with 3 buttons on it. But1, But2, But3
when i clicked on the But1 a new page link in opened (its and HREF),
when i clicked on the But2 a new page link in opened (its also an HREF)
when I  clicked on the But3 a new page link in opened (its an HREF)

All the three opened windows are different.
The thing is the user id are different for each one of the page opened  when clicked on the button.

if i click on But1 a new page opens with its userid. and If I donot close the window and click back button to go to Main.asp and click  But2 I am getting an error Access Denied. Because It assuming the same userid which opened the new page when clicked on But1. (the user can enter only from Main.asp choosing its appropiate Button)
If I close the Window , go back to "main.asp" and click on But2 it works fine but How do I make it work without closing the window.Just by clicking the Back button. Because My main.asp page is going to be kept opened always for users to select between But1 and But2 and But3 (ALL THREE ARE DIFFERENT USERS)

Also The important thing is I have do all the killing of the Session and Cookies in Main.asp only.
I cannot do any changes on the pages which are opened when clicked on Buttons.
I tried to clean the session and cookies on the page load of the Mai.asp so that when Back Button on the Browser is clicked
the main.asp is refreshed  but it does not work.
 
Also I donot know the name of the Cookies, How do i kill all the cookies.

Thanks