How to avoid page refresh until User click the button?

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C#

ASP.NET

Javascript

In my application a Captcha is coming at after every 12 minutes. In this there is no such validation applied to avoid page refresh, so because of this when Captcha come on the screen and user refresh the page and then captcha disappear from the screen without filling the Captcha details into the textbox. Now i want when the captcha come on the screen, User Can't be able to refresh the page until he submitted the captcha.

Hereis my javascript for captcha

function ShowNewPage() {
    var callbackFunctionArray = new Array(CloseCaptchaPopUp);
    modalWin.ShowURL('Captcha.aspx', 225, 290, 'Please enter characters dislayed in image to proceed', null, callbackFunctionArray);
}

Aspx Page code

<div id="Popup_content">
        <div class="box1">
            <div class="popupheadrow">
                <p class="title">
                    Please enter text below:
                </p>
            </div>
            <div class="bodyrow" align="center">
                <div class="row" style="min-height: 18px">
                    <asp:Label ID="lblMessage" runat="server"  style="color:Green; font-size: 0.8em;" Text=" "></asp:Label>
                </div>
                <div class="row">
                    <asp:Image ID="Image1" runat="server" ImageUrl="CaptchaImage.aspx" Style="width: 255px" />
                </div>
                <div class="row">
                    <asp:TextBox ID="txtimgcode" runat="server" CssClass="fields" TabIndex="1" MaxLength="10"
                        AutoComplete="false"></asp:TextBox>
                </div>
                <div class="row">
                    <asp:Button ID="CaptchaSubmit" CssClass="submit" runat="server" OnClick="CaptchaSubmit_Click"
                        Text="Submit" Height="26px" />
                         <asp:Button style="display:none" id="btnHidden" runat="server" OnClick="btnHidden_Onclick" />
                </div>
            </div>
        </div>
        <script type="text/javascript">
            setInterval(function () { btnHidden.click(); }, 60000);
</script>
        <script type="text/javascript" language="javascript">
            function CloseCaptchaModalPopup() {
                window.parent.modalWin.CallCallingWindowFunction(0, 'Information Saved Scucessfully');
            }
        </script>
    </div>

Suggest me some solution How could i avoid Page refresh until user submitted the captcha.