You could send a cease and desist letter if you believe the texts are in fact defamatory (see below), threatening legal action if necessary; or  you could simply institute legal action (i.e. sue the person for defamation).Defamation is the public making (to any third parties; so texts to clients would qualify) of untrue factual statements which damage a person's or business's reputation. Opinions are not defamamtion, even if very negative (e.g. it's not defamation to say, "it was a horrible place to work and I wouldn't trust them with my children"), and true facts are not defamation, not matter how harmful; however, if the texts contain  false factual assertions, that may be defamation, and you may have a cause of action.