as far as i know you can't do anything, because this problem you are naming is a option set in every browser. And this "error" oder Information is from MSIE ... i think. If the visitor has set IE up to display this pop-up every time, you can't do anything against it. Of course this error will not accure when you send that form to a non secure connection, because there is nothing to ask.
Maybe i am wrong, but as far as i understood you problem theres is no way ignoring the pop-up which is controled by the browser.
regards ali