how to override subscribe widget functionality?

I am using  Sitefinity 7.1.5213.0 version.I have added radcaptcha in subscribe widget of sitefinity.captcha works fine but even after captcha not valid ,the user get subscribed .And user gets added into mailing list .But i dont want to add email into mailing list when captcha not valid.I have added captcha validation on subscribe button click event. Below is my code.

Plz help me in it.

ascx markup:

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="SubscribeFormCustomTemplate.ascx.cs" Inherits="SitefinityWebApp.UserControls.SubscribeFormCustomTemplate" %>
<%--<%@ Control Language="C#" %>--%>
<%@ Register TagPrefix="sitefinity" Namespace="Telerik.Sitefinity.Web.UI" Assembly="Telerik.Sitefinity" %>
<%@ Register TagPrefix="telerik" Assembly="Telerik.Web.UI" Namespace="Telerik.Web.UI" %>

<%-- <script src="www.google.com/.../api.js" async defer></script>--%>
 
<asp:Panel ID="errorsPanel" runat="server" CssClass="sfErrorSummary" Visible="false" />
<fieldset id="formFieldset" runat="server" class="sfnewsletterForm sfSubscribe">
    <sitefinity:SitefinityLabel ID="widgetTitle" runat="server" WrapperTagName="h2" HideIfNoText="true" CssClass="sfnewsletterTitle" />
    <sitefinity:SitefinityLabel ID="widgetDescription" runat="server" WrapperTagName="p" HideIfNoText="true" CssClass="sfnewsletterDescription" />
    <sitefinity:Message ID="messageControl" runat="server" FadeDuration="3000" />
    <ol class="sfnewsletterFieldsList">
        <li class="sfnewsletterField">
            <asp:Label ID="emailAddressLabel" runat="server" Text='<%$Resources:NewslettersResources, EmailAddress %>' AssociatedControlID="emailAddress" CssClass="sfTxtLbl" />
            <asp:TextBox ID="emailAddress" runat="server" CssClass="sfTxt" />
            <asp:RequiredFieldValidator ID="emailValidator" runat="server" ControlToValidate="emailAddress" CssClass="sfErrorWrp" Display="Dynamic">
                <strong class="sfError">
                    <asp:Literal runat="server" ID="lEmailIsRequired" Text='<%$Resources:NewslettersResources, EmailIsRequired %>' /></strong>
            </asp:RequiredFieldValidator><asp:RegularExpressionValidator
                ID="emailRegExp"
                runat="server"
                ControlToValidate="emailAddress"
                ValidationExpression="[a-zA-Z0-9._%+-]+@(?:[a-zA-Z0-9-]+\.)+[a-zA-Z]2,4"
                Display="Dynamic"
                CssClass="sfErrorWrp"
                ErrorMessage="<%$ Resources:ErrorMessages, EmailAddressViolationMessage %>">
                <strong class="sfError">
                    <asp:Literal ID="lEmailNotValid" runat="server" Text="<%$ Resources:ErrorMessages, EmailAddressViolationMessage %>" /></strong>
            </asp:RegularExpressionValidator></li><li class="sfnewsletterField">
            <asp:Label ID="firstNameLabel" runat="server" Text='<%$Resources:NewslettersResources, FirstNamePublicForm %>' AssociatedControlID="firstName" CssClass="sfTxtLbl" />
            <asp:TextBox ID="firstName" runat="server" CssClass="sfTxt" />
        </li>
        <li class="sfnewsletterField">
            <asp:Label ID="lastNameLabel" runat="server" Text='<%$Resources:NewslettersResources, LastNamePublicForm %>' AssociatedControlID="lastName" CssClass="sfTxtLbl" />
            <asp:TextBox ID="lastName" runat="server" CssClass="sfTxt" />
        </li> 
        <li>
        <%--<br /> 
        <div class="g-recaptcha" data-sitekey="6LfNuhgTAAAAAHoVGH51Vs3iYNRb2Dual2_8Tv26"></div>
        <br/>--%>
        <asp:Label ID="lblMsg" runat="server" Visible="false" CssClass="sfTxt" />
</li>      
    </ol>
    <div class="sfnewsletterSubmitBtnWrp">
        <asp:Button ID="subscribeButton" 
            runat="server" Text='<%$Resources:NewslettersResources, SubscribeToList %>' 
            CssClass="sfnewsletterSubmitBtn" onclick="subscribeButton_Click" /></div>
</fieldset>
 <asp:Panel ID="selectListInstructionPanel" runat="server">
    <asp:Literal ID="pleaseSelectList" runat="server" Text='<%$Resources:NewslettersResources, ClickEditAndSelectList %>' />
</asp:Panel>

Hi Rupali,

Please check the below KB article for more details about how to extend the subscribe widget and add a captcha control

http://www.sitefinity.com/developer-network/knowledge-base/details/extend-subscribe-form-widget-with-server-side-captcha-validation

You may also check the below forum post where I have added some additional details on how to implement it and also have attached a video demonstrating the exact steps;

http://www.sitefinity.com/developer-network/forums/bugs-issues-/captcha-in-subscribe-widget#RgbJGkiYP0WgqoyIbjeE5g

Regards,
Sabrie Nedzhip
Telerik

Thank You for your reply...I will try this solution.