cancel button_click event

Hi,
I want to cancel the submit event when the validation is false but it doesn't work. How do I do it?
protected void Button1_Click(object sender, EventArgs e)
        {
            if (Page.IsPostBack)
            {
                ccJoin.ValidateCaptcha(txtCap.Text);
                if (!ccJoin.UserValidated)
                {
                    some code here
               }
   ...