Limiting the number of checked checkboxes

I have the following code in Javascript that alerts me when 4 or more checkboxes have been checked, but what I would like it to do is to not allow any more checked checkboxes after 4 have been checked and to disable the checkboxlist/submit button etc. My code does not disable these elements :(

Can this be done from code behind???

<asp:Content ID="Content1" ContentPlaceHolderID="Head" Runat="Server">
    <script type="text/javascript" language="javascript">
        function CheckCheck() {
            var chkBoxList = document.getElementById('<%=cblTopics.ClientID %>'); 
            var chkBoxCount = chkBoxList.getElementsByTagName("input");
            var btn = document.getElementById('<%=btnSchedule.ClientID %>');
            var i = 0;
            var tot = 0;
            for (i = 0; i < chkBoxCount.length; i++) {
                if (chkBoxCount[i].checked) {
                    tot = tot + 1;
                }
            }
            if (tot >= 4)  {
                alert('You may select up to 4 topics per meeting.'); 
                btn.disabled = true;
            }
            else {
                btn.disabled = false;
            }
        }
    </script>
</asp:Content>

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