Hello, I am a solicitor with 20 years experience I will try to answer your question.
It is not harassment to report someone to the police/HMRC for crimes they have committed. However you must avoid threatening to report them and demanding they do something to avoid being reported. This could amount to blackmail which is a serious criminal offence.
That is not harassment. There is no risk of a valid complaint being made. It is highly unlikely that the person concerned would take the risk of reporting you and inviting scrutiny of their own affairs. I was just warning you of a line you must not cross.