Sometimes taking a different approach can work.
I noticed someone marketing themselves on social media using others watermarked images and implying they were samples of their work. Because this person did not know me, I was able to respond as a potential customer, and politely play dumb. "I'm confused. An image you are posting here is watermarked xyz photography, another is watermarked abc photography. I don't quite understand, did you actually shoot these photos?" The response back was "No, these are just representative of a type of shoot I COULD do." To which I replied, "Oh, I see. Well, could you provide us a link to some of your work? Thank you." The response, "My website is still under construction."
Yeah, politely busted for all to see! The key is the person in question must not know you, or that you are trying to attack them.
Some folks doing this stuff may have good intententions, but are clueless to the fact that this could mislead the customer, and could possibly land them in hot water if the photographer found out. Without an explicit explanation that the work shown is not theirs they are using questionable methods to market. On the other hand, others know exactly what they are doing and simply don't care.