Radio buttons would only need to be read-only if there are other options. If you don’t have any other options, a checked radio button cannot be unchecked. If you have other options, you can prevent the user from changing the value merely by disabling the other options:
<input type="radio" name="foo" value="Y" checked>
<input type="radio" name="foo" value="N" disabled>
Fiddle: http://jsfiddle.net/qqVGu/