Oops, sorry, I misunderstood. I see now what you're asking.
Hmmm, good question. Well, there are two options I can think of:
- Instead of having a radio button for each option, you could have a select list where you would specify the default value, and the list would have an option of "No default" (or "None", or whatever you want there).
- A checkbox at the top of the form that would automatically clear the Default radio button settings (similar to what you see on webmail apps for selecting all items in a list). This could be either an AJAX thing, or something that happens on page submission; when you submit the page, the default setting for the options is deleted from the database so that when the page is displayed the next time, the Default setting is gone.
My preference would be #2, since that's the cleaner way to do it.
If you want to point me in the direction of where the code is that handles this, I'd be happy to work on modifying it myself and submitting a patch.