I did catch another FReeper that Posted to you explaining that the Security Guard took nothing from the Illegal Invader that attacked him, so that point is mute.
The New York statute that authorizes the use of force to stop a larceny doesn't require that the person using force be the victim of the larceny (which makes sense - for example, if you witness a purse-snatching and then tackle the guy running away with the purse, you're protected by the law). So, the fact that the sign was stolen from someone else in the group, rather than the guy who tried to get it back, doesn't matter.