Set radio button to no initial value
Add the below code to a custom HTML block, update the name in bold to the name of the field.Replace the **** with the word "script"
<**** type="text/javascript">
(document).getElementsByName('FIELDNAME')[0].removeAttribute('checked');
</*****>
Please ensure that you have access to an experienced JavaScript developer.
Marketo Technical Support is not set up to assist with troubleshooting JavaScript.

Here's a sample page: info.dbmayberry.com/noinitialbutonvaluechecked.html