Ironically not even the owner of the theatre (who will probably have more interest in copyright infringements than the audience has) is legally entitled to insist a photographer deletes any photographs which s/he suspects are copyright infringements. The owner may request deletion but the photographer may choose not to comply. Handling the photographer's camera leaves the owner open to charges of assault. The most the owner can do is request the photographer stops, or require the photographer to leave if they will not stop.
In any case deleting photos from a memory card is mostly pointless since under the right conditions they can be recovered later on.
Oops, Classic got there before me!