and the insight I had was that someone should stop the kids and point out that *they* may be having fun but the kid the item belongs to *isn't*. If he had a reasonable chance, then it *might* still be considered play. but as he doesn't it's turned into bullying.

IME, they know that. That's what makes it fun for them - the fact that the person they stole the item from isn't playing.

(If everybody is playing, I called that "monkey in the middle". That's got rules, though.)