You should be able to set a rule. Radio buttons are generally several buttons bound to the same field. Check the properties of each so you know what the value of the field will be when one is selected. Or temporarily put a text box or expression box bound to the same field as your radio buttons on your form so you can see what the value is. Then, set a rule on the field with conditions: i.e., if field1 is equal to 1 (or whatever your radio selection values are), set field2 to blank.