Something similar happened to me, sorta similar anyway.
About 6 yrs ago i got an angry message from someone on facebook asking why i had not sent them the lens i sold them. I had no idea what they where talking about, so after a lot of investigation my wife and i found our facebook account AND our paypal accounts had been hacked and someone had sold a canon lens to about 15 different people. Our paypal account showed over $11,000.00 in transactions over just 3 days. We of course reported it to both facebook and paypal (all the money had been drawn out within minutes of each transaction.) They both found in our favor. We closed those accounts.
5 yrs later i was attacked on facebook claiming i had ripped them off a few yrs ago and they wanted there money back. After explaining it all to him he did not believe it, (not sure i would have either). Anyway he wound up being the guy that started it all yrs earlier. After much investigation by paypal facebook and the local Portland police, this guy was tracked down, we where informed just last month he had been found and arrested. From what we where told he had over a dozen other scams he stole over $75,000. And he was broke again so he was going back through facebook finding the people that he had hacked trying to strongarm them into paying him.
Crazy a** people out there.

Exactly!!! Example, big photo target near me, KSC - Vehicle Assembly Building or VAB.... one of largest buildings in the world (it's HUGE, had it's own inside weather issues in the early days) no one who works there, or tours there, or lives within a few miles of it, doesn't come away with a pic, heck you can take a photo of it from Titusville, or the Port, Cocoa Beach, if you are above the tree line..it is like that for any point of interest/monuments/lakes/buildings/canyons, yadda, yadda yadda. Hmmm, Mount Rushmore comes to mind, Empire State Building, Grand Canyon, etc. etc.

Ignoring them is probably best, or simply tell them to look at the Exif data (set up your camera to tag your pics with name, or whatever, watermark), although that may not transfer to social media.

Wow, just had an upsetting situation happen on Instagram. I made a comment on a non photography post on Instagram and someone must have gone to my page, or what ever you call it, and looked at my photos. They then went on to say that many of the photos were not mine! They even said they had seen one on Nat Geo and was not mine and I was not giving credit! I responded to this person saying they were certainly all mine and the response back was there is no watermark and why do others names show up? Then proceeded to ask if I worked for Nat Geo.... I can only assume the other names they referred to are the people I tagged who were on the trip with me and I do not use a watermark, as many on Instagram do not.

This just really upsets me to be accused of claiming someone else’s photo! I did once again reply that I guaranteed that 1,000% they are all my photos and I was honored they thought my photos were Nat Geo worthy....

Wow, just had an upsetting situation happen on Instagram. I made a comment on a non photography post on Instagram and someone must have gone to my page, or what ever you call it, and looked at my photos. They then went on to say that many of the photos were not mine! They even said they had seen one on Nat Geo and was not mine and I was not giving credit! I responded to this person saying they were certainly all mine and the response back was there is no watermark and why do others names show up? Then proceeded to ask if I worked for Nat Geo.... I can only assume the other names they referred to are the people I tagged who were on the trip with me and I do not use a watermark, as many on Instagram do not.

This just really upsets me to be accused of claiming someone else’s photo! I did once again reply that I guaranteed that 1,000% they are all my photos and I was honored they thought my photos were Nat Geo worthy....