how to clear previous page's data?

Question

hello i am developing e-shopping website
after adding items in cart.aspx...n by clicking on "FINALE PURCHASE" button control goes to the card.aspx
--->click method of cart.aspx

 
protected void final_Click(object sender, EventArgs e)
{
Session["total"] = lbltot.Text.ToString();
Response.Redirect("card.aspx"); 
}

 

 
card.aspx
 
by clicking on submit in card.aspx

 

 
protected void submit_Click(object sender, EventArgs e)
{
Session["cno"] = cno.Text.ToString();
Session["pwd"] = pwd.Text.ToString();
Response.Redirect("balance.aspx");
}

the balance.aspx will show the detail of credit card.. that is card no and available balance... and button for "final purchase"
by clicking on final purchase button in balance.aspx.. it will transferred to "Final.aspx" where the balance is deducted from account..
 
now from final.aspx when user clicks on back button of the web browser it shows the data of balance.aspx.. which should not been shown..
 
in php this problem can be solved by taking form as method="post" bt in this it does not allow to take 2 form tags run at server..
i have take

 <form>

in masterfile.master bt in balance.aspx it dosnt allow to take