It is bullying or a form of harassment – the end doesn’t justify the means, however frustrating you may find someone’s behaviour.
Anyone who thinks it is ok to send a demand for money with no legal basis, coupled in this case with an “interest charge” should know that that is fraud, i.e. a criminal act. It is the same as demanding payment of a fake tax debt. Stop it immediately, or you may find yourself in quite a bit of trouble.