Answer:
The only really ethical way to hurt a sociopath is by exposure. They are usually quite arrogant- often to the point of believing that everyone else is stupid. Sooner or later, they overestimate their skills and underestimate other people's intelligence. They get caught in their lies or their bad behavior gets discovered. You can help this whole situation along by researching their pasts and investigating what they are up to at present. An anonymous note to someone significant to them ( boss, lover, future victim) may thwart and dismay them. These people frequently have a lot to hide and go to great lengths to keep unfavorable information from seeing the light of day.
Just be very careful about any actions you take. An angry sociopath can cause you a lot of problems! (Mine actually injured himself in an attempt to frame me for a felony assault ! - although he didn't get away with that, in the end I had to leave town because he was clever and very determined to hurt me . He also manipulated half the police force into thinking that I was stalking him, when in fact, the opposite was true. However, eventually, I did have my revenge. A couple years later, he jumped probation and fled the state in order to avoid a felony charge. I tracked him down, alerted the authorities and did not let go until he spent the summer in jail. He didn't even know that I did it or where to find me at that point,)
Remember, you are dealing with a person who has no qualms about hurting you badly so exercise great discretion!