Just leave a comment pointing out that the pictures are of someone else. Like there's a girl on here now who's using pictures of a country singer Danielle Bradbury and when you google search the images it actually links to the singer's twitter which is amusing.
I usually play them along for a while and then let them know that I know they're full of shit. Which is when most of them block me.
Because of the posting rules against "member posts" - you can't really say much about other users. And there's no GaG site rule against being a catfish - even though the user agreement says that you agree that you are who you say you are.
It's a bit ambiguous at best.
So I play along until they bore me, then I tell them I know..🙋