Summarize this article in one sentence.
Choose the important issues to argue. Don't try to prove what he says isn't true, because it is nearly impossible unless he makes specific accusations. Just say it isn't true and let it go. If your history is full of trustworthy examples and integrity, people will believe you. If someone in your mutual circle is helping spread lies, confront them directly, but without emotion, and ask them to stop gossiping and spreading falsehoods. This will only make you look the same in the long run. Is better to stand up for yourself with integrity and resist gossiping about your ex. Make amends if possible. Doing so gives you more credibility when refuting untruths and reminds others that everyone is human and makes mistakes. Do not act in order to reach a specific outcomewhether to hurt him back or to get others on 'your' side. Your first priority should always be what is best for you. In each instance, explore the consequences of every option and choose either the one that has the fewest negative ones or the most positive ones for you. For example, if he is trashing you to mutual friends, think over your possible reactions and your friends might take them before blurting something out loud.
Don't try to refute or counter every untruth. Don't trash talk your ex. Own your mistakes. Make sure your actions are not just reactions.