Take a look at this post: http://tiffinbox.org/the-new-wedding-guest/
It talks about guests with cameras at a wedding.
The article is so true and I see it all the time at weddings. Guess there is nothing you can do about it
Actually I got a very good example I can share. The client walking down the isle with the guests all seated on the sides and ready to shoot some streamers. I had my wide lens on so I could capture both the clients and the guests with the streamers flying at them. As the client got to the start of the red carpet a guest decided to take his dslr out and stand right in the way. So now I got 10-15 images of him in it while everyone else is seated!!! Lucky i could crop, but now for the shot I loved I cant include the guests in it as its ruined!!
I was pretty angry that whole night