No, there is no way to prevent her from calling: *anyone* can call your employer and look to verify your employment (e.g. landlords often do this, to make sure you can pay), so certainly a PO can. (The employer can choose to not respond or comment, but the other person has the right to call and ask.) You might want to ask the PO if there is some evidence of employment she would accept other than calling the employer: e.g. a paystub.