Without a noncompete agreement, can we stop a former employee from undercutting us?

Q. We just had a successful salesperson quit his job and join one of our major competitors. We did not, unfortunately, have him sign either a noncompete agreement or a confidential information agreement. We are very concerned that he may have taken, and may be using, some of our company’s confidential business information, including detailed customer information. Is there anything we can do about this situation, given the absence of any written contract?