Scum of the earth, scam artist, do not answer

This company flew into town, cleaned the filters in our restaurant, rang for payment the next day which was made. 8 mths later I paid $600 odd dollars into his account in error and have been chasing its return ever since. Phone calls, emails, text messages - no contact at all. Last email I sent threatened legal action which I doubt will get me no where. He was supposed to return for a yearly service - guess what! Lousy for small business.