It is not possible to set the access property of an individual radio button at run-time in Acrobat or Reader 9, or Acrobat or Reader 7. This was possible in Acrobat/Reader 8.
Set the property on the exclusion group level (that is, the parent object) and not on the field level. For radio button lists, set the property at the Radio-Button group object and not the individual button object.
From Designer 7.1 Help:
About Radio buttons
Radio buttons are contained within exclusion groups. Only one radio button in the exclusion group can be selected at a time.
An exclusion group can be manipulated as one object. Some options on the Object palette apply to all of the radio buttons in the same exclusion group.
Radio button properties in the Value tab
When you create an exclusion group of radio buttons, the Value tab presents a number of options that you can apply to the entire group.