Like many legal questions, a great deal depends on what state you are in.  But, generally, in Virginia, the answer to this question is “Yes, you can.”  The analysis is fairly simple: the contract is not with the employee, it is with another business.  You are not limiting (except indirectly) the employee’s freedom to work, you are freely contracting with another business to provide a service and part of your price is that the other business not destroy yours.  There are exceptions and limitations to this rule, however.  For example, the entire U.S. Government is not considered a single customer.  Another example: If the employee’s skills are so esoteric that only 1 or 2 firms could possibly employ him, it is possible that the agreement would be likened to indentured servitude and struck down.  So care is required.