It's only ever happened to me once.  I saw it from a distance and realised what had happened (photo was at least 5 years and 100lb ago)  I could have just walked away but decided that this would be too easy. I walked up and in the middle of a crowded cafe asked ObeseWhate86? (not it's real name on dating site, I have long since forgotten that) Just so people knew what was going on and she sweetly said yes with a big smile on her face.  I said "OH HELL NO" loudly, turned around and walked out. No idea what other peoples or her reaction as I didn't look back or if she went to write or text back as I immediately blocked her number.  Fat fucks that do this shit need to be publically humiliated, it's doing a public service.