They can "tell" you anything they like. However, they may only enforce this no contact demand against you IF you signed some contract or agreement, including a separation or severance agreement, in which you agreed to certain terms (like no contact) in exchange for something (such as payment of severance or a bonus). If you did not contractually agree to this, they can't enforce it against you--though they could enforce it aganst their employees (e.g. they could fire, or do anything less than firing, like suspending, disciplining, transfering, demoting, etc.) an employee who has contact with you.