Prevent hiding of ModalPopupExtender when ok or cancel is clicked

function conditionalHide(clientID)
{
    if (confirm('You sure?'))
    {
        $find(clientID).hide();
    }
}

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        btnOK.OnClientClick = string.Format("conditionalHide('{0}'); return false;",
            panPopup_ModalPopupExtender.ClientID);
    }
}