Bad mouthing my company on social media to get a payment from a year ago

The director from Club Cocomo has contacted my agency for payment from April 2019. When asked about a voucher received from the agency I was told she was given it telephonically All they have is an email from one of our agents I asked how many times did they try and contact us for payment- I have had no reply to that This evening she posts on our Facebook page basically calling us thieves. We don't have the client any longer and had they followed up earlier perhaps we could have checked regarding the booking. I understand completely how frustrating it is to try and get money outstanding but we too have government agencies that owe us money and after numerous emails requesting to book if we don't follow up for payment or don't receive their POs which are essentially their " vouchers" we get no payment. No client will pay us a year later. Don't bad mouth anyone when you haven't done your job just to scare clients into payments Not on