It's not 100% fool proof, drag and drop and get around it, but OnObjectEntry can perform a script that checks the value of the radio button field and then uses go to field, go to object or go to next field to put the focus somewhere other than the field just entered to deny access.
You might back this up with a validation rule that rejects all input if the right value isn't selected in the radio button. That way, the script trigger keeps things user friendly while the validation rule makes sure that user's can't accidentally get around it.
Thanks Phil. Good for me. I'm guessing there is no way to script a prevent field entry like you can select in the Inspector...
Correct. That's why I usually set up a validation rule to guard against "accidents" that might modify the field when it is not permitted to do so.