Unless you provide for this in the contract of employment, you can’t really (or, it’s very difficult to…unless it is a criminal offence)!
You can include post-termination restrictions (restrictive covenants) in the contract of employment, which, by signing the contract, your worker agrees, for example, not to work for competitors, not deal with your clients, or not solicit your staff for a fixed amount of time after their employment ends. Care needs to be taken as to the length of time applied to such restrictions and legal advice needs to be sought to ensure they are going to be enforceable.
If your ex-employee breaches any restrictions, you could apply to Court for an injunction and/or damages. However, restrictive covenants must be reasonable and they must not go any further than protecting legitimate business interests.