How to deal with this kind of thing?

first, I suggest that the company tell the manager about this matter, and tell him that the company trusts him and lets him work with confidence. The company speaks with evidence. And give him some consolation money. After that, I will never pay attention to this matter again. Generally, people who send text messages are self-defeating, and often it will pass after a while.

If the party concerned can't accept it:

Second, call the police or hire a professional organization to pursue it (to raise the estimated loss to a certain level to alarm the police investigation), it is almost impossible to pursue it by more advanced means.

If it is really impossible to find out, there are two alternatives:

1. (The brother who has been rumoured has to resist greater pressure) Hold a company meeting, make public the rumor by short message, claim that the police have been alerted (whether the police investigate or not, scare the police first), and be sure of the work performance of the rumourer. Everything is based on evidence (paving the way for the brother to have something to do in the future), indicating that no matter how to send a rumor message, it will not be sent.

2. Let the rumored brother make a choice, whether he wants to resist the pressure and take the consolation money, or choose to transfer from a department or post. The reason is probably that he has enemies with the same department or subordinates or post-related personnel. It is best to choose to continue to work, and the transfer of posts will make other colleagues in the company follow suit.

finally, there is a scheme that can be tried, but it needs more cost from the company. That is to change the post of the rumoured person into a double-post system, and he will still be in charge of all decision-making and foreign exchanges, but a nominal "supervisor" will be added. This "supervisor" will supervise all key events made by the rumoured person, but will not make decisions. It is necessary to make it clear to the rumoured person that this gesture is for the rest of the company, not to distrust him. If the text messages continue to be sent, then everyone will understand that this text message is a rumor message, because people who make enemies are basically out of revenge and generally don't stop until they get what they want. If they make enemies, they will probably continue to send them to aggravate the slander. If the short message stops, everything will be settled. After a while, the "supervisor" will only appear in the name of supervision, and it will be fine to transfer from doing other things in a practical way. Here, don't doubt that the client really has a problem just because there is a supervisor, to prevent the sender from being smarter, and the criterion for judging whether the client has a problem is not whether he is selfish or not, but the profit ratio of his performance and cost.