I had a guest whose profile photo gave me a bad feeling, even though our messaging through the Airbnb platform was respectful -- perfectly okay, in fact. When he checked in, we had a discussion about Airbnb in general, in the course of which I very nicely suggested he think about changing his photo, for all the obvious reasons. He smiled and nodded, and off they went to explore the city.
He and his girlfriend TRASHED my house. They left leaking trash bags in the kitchen, a disgusting mess in the bathroom and grease on every surface. He gave me my only 3 star review to boot. In retrospect, I should've been harsher in my review, but I did mention the mess.
The more I thought about his disrespectful behavior, the more his photo bugged me. It should have been a big red flag, and I should have trusted my gut feeling. So I flagged it on his profile. That was many months ago. Just checked, and it's still there.