One solution that I have seen others use is to allow respondents to clear a radio response by clicking on the selected item again. You may need to explain to respondents that this functionality exists as it is not a standard of radio buttons across the web.
If this option sounds good, this is the script I have used in the past:
<script>
$(document).ready(function(){
var resps = {};
$('input[type="radio"]').each(function(){
var name = $(this).attr('name');
resps[name] = SSI_GetValue(name);
});
SSI_CustomGraphicalRadiobox = function(graphical, input) {
var name = input.name;
var resp = SSI_GetValue(name);
if (resp == resps[name]) {
SSI_RadioReset(name);
resp = 0;
}
resps[name] = resp;
};
})
</script>