I have heard this asked many times by cuckolds and wannabes. Difficult to do to be honest. Some medications can make you impotent but you will need medical supervision; castration is another course but has many many nasty side effects; also being in chasity for a lengthy period of time (e.g. weeks or months).
Have not done it myself but like becontree2001uk's idea of using a chasity device. If she locks you in it then keeps the keys in a safe box that only she knows the code to you would be rendered totally impotent and she would be free to breed with whomever she chooses!