You *may* be able to bring a legal action against him for theft of business information--your client list--and seek an order preventing him from soliciting your clients, but this is, frankly, not likely to work if the assistant did not sign non-competition, non-solicitation, and/or confidentiality agreements. That is, to establish that someone cannot compete, or solicit, or even use your client list for him/herself, you generally need to  establish some legal basis for that by having the person execute an agreement to that effect. While you may wish to discuss this matter in more depth with an attorney (many will provide a free initiail consultation), it may well be that you could spend a fair amount of money pursing a cause of action taht will ultimately fail.You may need to absorb this loss for now, but in the future, *always* have assistants sign non-compete, non-solicitation, and confidentiality/non-disclosure (e.g. will not use your client list or other business information for own benefit) purposes.