Default terms based on user role
There might be better ways to do it, but this what I did in my case. 


In hook_form_alter loop through the '#options' for this field.
Store every item in a new array skipping the one you don't want as options for this field.
 (in my case it was the item at index 0 with value 'N/A').
Assign this new array of options as '#options' for the field.


Worked for me.