I ran in to that last week where a guy paid for BS to "save" the entire emergency exit row for his family. I had a little fun and played like I didn't understand for a minute to give him little bit of a hard time. "Sorry, which seat were you saving? The isle or the window? Both? Oh, sorry, I'll go to the other side of the plane. Oh, you're saving one of those? Which one, isle or window? Both? Wait, I thought you were saving both on the other side.... Hold on, which window is free? Neither? Oh, I'll take this isle seat then. No? I'll go over to the other side then... That's the free one, right?"

It was only an hour flight so I wasn't too peeved about it.