"Smooth" means "suave" in this context. "suave suggests a specific ability to deal with others easily and without friction - a suave public relations coordinator" So, the criminal is not only good at what he does, but he also does it in some kind of sophisticated/stylish way. M-W: "Smoothly though often superficially gracious and sophisticated" Well, and smooth because he hasn't been caught. "Everytime I try to find him/ he's leaving no clues left behind him/ And they have no way of knowing/ of the suspect, or what to expect."

It means Eliminating criminal smoothly depending on the power of justice.