If the employee who has left is in sales or any other confidential position dealing on your company's behalf to your clients, you need to give a public notice alongwith the empoloyees photograph stating that he has left the organisation and no longer represents your company and that anybody dealing with him for your company shall be doing so at his own cost and consequences.
I think this caution notice should serve the purpose. In addition you have the facility of email wherein this notice can also be forwarded to all your clients.
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