Be careful about making a “public” warning. It can be used to sue you for defamation. I know it’s tempting to do but the courts take a dim view of public opinions. Public opinions are records that can be used in court. The guy may be the worst mechanic in the world and a thief to boot but don’t lose your financial future because of it. If you don’t have a court judgement against him your “case” is just an opinion. An opinion is not proof of wrongdoing.