I think you would have the option of surrendering the item, or leaving the venue.
I don't think they could just take it.
As a side note.
In the vast majority of venues you are better off without the flash anyway.
The object you are trying to photograph is most likely out of the range of the flash.
If you disable the flash, and rely on the stage lights your picture will turn out better.
This was the advice given to me by a friend who takes photos at concerts professionally.