The best way to respond to a girl that says she has a boyfriend is to back off. You never know if she really has a boyfriend, or whether she is trying to test your reaction, or if she is simply pushing you away.
Nevertheless, there is not much that you can do in this scenario and hence, the best thing to do would be to back off immediately and to stop bothering her with your calls and texts. This would be the most civilized reaction that you could give as an adult, to someone who is already in a commitment.