Radio buttons actually mean that either one of the values is to be selected. The "unselected" state is actually not really a "legal" state for radio buttons. However, you can reset the radio buttons just as any other fields if you have not set one of its states to be the default.
The value of the "unchecked" state is always "Off". So, using JavaScript, you set the value of the radio button field to "Off", and it will clear the selection, such as in
this.getField("myRadioButton").value = "Off" ;
or by simply resetting, such as in
this.resetForm(["myRadioButton"]) ;
In new forms (because nowadays, changing the field type is a big bother (you delete the field and add a new one of the new type, and then try to fix the tab order), you can make things easier for the user by using checkboxes. If you create a set of checkboxes, with the same name but different return value, you have mutually exclusive settings, but the user can simply uncheck a checked box. If you set the values via JavaScript, you will do it the same way as described above.
Hope this can help.
Max Wyss.