Clearing webform after I refresh after webpage Expires?

I have webform FillData.aspx with two textboxes, a checkbox,  and a Submit button. This is the basic functionality: when I click on the checkbox, it copies the data from one textbox to the other. When I click on Submit, I redirect to page Finish.aspx. Since FillData.aspx expires immediately, when I click the "Back" button in the browser in Finish.aspx, I get the "Webpage has expired" page, which is good.
The problem is that when I refresh the page, not only do I get the data that the user had typed in, it also executes the code in CheckBox1_CheckedChanged.
My question is: How can I clear the typed text after the user refreshes when he clicks on "Back"and sees "Webpage has expired" page?
Here's my code: All my code is server-side. I also tried overriding LoadViewState and SavedViewState, but it didn't work.
protected void Page_Load(object sender, EventArgs e)
{
        Response.Buffer = true;
        Response.ExpiresAbsolute = DateTime.Now.Subtract(new TimeSpan(1, 0, 0, 0));
        Response.Expires = -10000;
        Response.CacheControl = "no-cache";
        if (!Page.IsPostBack)
                //Clear textboxes
}
protected void Button_complete_Click(object sender, EventArgs e)
{
        Response.Redirect("Finish.aspx");
}
protected void CheckBox1_CheckedChanged(object sender, EventArgs e)
{
        if (this.CheckBox1.Checked)
                // copy text from one textbox to the other.
}


Thanks.