You speak to a solicitor for advice. A letter can be sent to the vile person by the solicitor.
If you don't know who said it then you can't have a letter sent. About people believing rubbishy stuff depends on where you live. There are times when it takes some years for people to come to know the liar and cheat. There are also those counties that prefer to keep the rubbish and continue to spread it. This is the result of terrible ignorance and brought up in abusive situations, so the mind cannot accept there is another way to think.