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It is only possible if you download your form source and disabling the button since the radio button does not have a read-only attribute. Basically, it depends on what you are after to, making checkbox or radio buttons a read-only does not restrict your user from interaction with the button but disabling it will so depends on what you are trying to accomplish here.
Please check the discussion in this page http://stackoverflow.com/questions/1953017/why-cant-radio-buttons-be-readonly why the fields that has selectable option does not allow readonly. You can also check the difference between disable and read-only here http://www.faqs.org/docs/htmltut/forms/_INPUT_DISABLED.html.