Write a letter informing clients about termination of Services Salesman.

Dear Sirs 

This is to bring to your kind notice that our traveling sales representative Mr. _________ has left our services with effect from (Date). Therefore, Mr. _____ is no longer authorized to book orders, collect payments or make any kind of commitment on behalf of our company with immediate effect. If, any one dealing with him will do so at his own risk. 

Yours faithfully

Manager