It will be very difficult to prevent a former employee from moving to a competitor. Courts and the law are generally hostile to the idea of restricting a person's freedom to work. You can possibly prevent the employee from taking a similar position at the company, where the disclosure of trade secrets would be inevitable, however. A noncompete agreement would be helpful, if the former employee agreed before working for you that she or he would not take a similar position where trade secrets would be disclosed. You need to consider carefully the nature of your trade secrets and how protected they are, and how specific the terms of your noncompete agreement are - the more specific the better.