How do you go about being a whistleblower on people you work with ?

Two of the people we work with, are having an affair, usually at the workplace, and the woman is cotinuously stealing things and giving to him and his family, to keep the affair going, how do we tell on them with out being a big bi**h ?

Go to the person highest up, explain what is going on and that you want to be anonymous. Also that you have witness that feel and wish the same. That way no-one knows it was you. And the big wig looks like the bad guy (as always). :)