There isn’t really any way to keep a narcissist from talking bad about you, especially if they are angry with you or in the ‘devalue’ phase of the relationship cycle.
You know the scene in every cop movie where they say “Everything you say can and will be used against you,” Well, that’s a relationship with the Narc, in a nutshell. They are cop, judge, jury and executioner to you, their offender, persecutor, and prisoner. Everything you have ever said or done in front of them, every secret you’ve trusted them with, will be used against you.
The fun part is, even if they don’t have any dirt to use against you, they’ll just make something up. You remember that trip out of town you took last summer to visit old college friends? Well, the nugget of truth “she was out of town for two weeks” gets a sick and evil story spun around it, “she went out of town to get an abortion and get treatment for syphilis,”….and because there’s a little nugget of truth in the middle, it makes the whole story easier for others to believe. Even if they know absolutely nothing about you at all, they’ll make up a story to make you look bad and make themselves look good, or make themselves look like your pitiful victim.
There isn’t really any way to control what a narcissist says about you, ever. The best thing to do when you realize you are dealing with a toxic crazy person is to cut ties, go no contact, and try very hard not to react or respond when these vile tales of things you supposedly said and did come floating back to you. The less you respond and take it to heart, the sooner the narc will get bored with badmouthing you and move on to a more interesting target.
Good luck!
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