Dear Mr. Karthick,
If the employee who has left is in sales or any other confidential position dealing on your company's behalf with your clients, you need to issue a public notice along with the employee's photograph stating that he has left the organization and no longer represents your company. Anyone dealing with him for your company shall be doing so at their own cost and consequences.
I believe this caution notice should serve the intended purpose. Additionally, you have the option of using email to forward this notice to all your clients.
Regards