What should I do if someone uses text messages to maliciously slander another person’s reputation?

An act that does not constitute a crime is a civil tort:

According to the "General Principles of the Civil Law", citizens enjoy the right to reputation. Since they enjoy the right to reputation in accordance with the law, this right is protected by law and shall not be tolerated. Anyone who wants to infringe.

Article 110 of the "General Principles of the Civil Law": Natural persons enjoy the rights to life, body, health, name, portrait, reputation, honor, privacy, marital autonomy and other rights.

Second, if the circumstances are serious, it constitutes a crime:

If you use various methods to maliciously slander the reputation of others, and the circumstances are serious and cause certain consequences to others, you can file a lawsuit in court and request to pursue criminal liability.

Article 246 of the "Criminal Law" Crimes of Insult and Defamation Whoever openly insults others by violence or other methods or fabricates facts to slander others, if the circumstances are serious, shall be sentenced to fixed-term imprisonment of not more than three years, criminal detention, or public surveillance or deprivation of political rights.

The crimes in the preceding paragraph will only be dealt with upon complaint, except for those that seriously endanger social order and national interests.

If the victim complains to the People’s Court about the conduct specified in Paragraph 1 through the information network, but it is really difficult to provide evidence, the People’s Court may request the public security organs to provide assistance.