In these days of mobile phone cameras & recorders, there's very little anyone can do about it, unless you spot it.
I think it's the height of bad manners, incidentally, to photograph or record anyone without asking.

I was playing in a small pub for most of the summer; a 60 seater, where I'd be doing an hour or so, every night. I normally have my eyes closed when I sing (... that has gotten me into enough trouble over the years in itself...), but open them every once in a while during the space of a song (to see if anyone has a gun pointed at me).

I spotted someone 'acting suspiciously' (as we say in my neck of the woods...), and saw that he was trying to put a mini-disc into his wee recorder.

I just stopped the song, and looked at him til he turned around to see what was up, and then I ripped strips off him for being sneaky. I told him if he had have asked, and done the normal human moral thing, I'd have plugged his mini disc into the 'Tape Out' of the desk, for him

Lifting your instruments without permission is a NO NO

B.