Cell Phones in public places

I have been trying to think about what I can do to people who talk on their cell phones loudly, when I am close by.? I used to want to go up ,stand next to them with an imaginary phone in my hand and say bla bla bla bla really loud to see if they get it.
Now I have a new idea, when they are annoying me with their loud conversations on their phones, I will go up to them with a microphone, record them talking then get off the bus with them and play it back on a giant boom box, while I follow them home .