I initially very politely asked them if they could stop talking because we're trying to watch the movie, but they didn't take that very seriously.
After another ten minutes of their commentary I yell very loudly "SHUT THE FUCK UP!". Extremely loud, I suspect everyone in the theater heard me pretty clearly. I'm a pretty big guy and I have a very loud and deep voice, and of course the theater is dark, so they might have assumed I was more threatening than I actually am. The teenagers shut up for the rest of the movie.
The thing is, though, it kind of ruined the rest of the movie for me. The entire time I'm sitting there, kind of worked up and annoyed that I had to yell at some kids and ruin their Friday night.
I've certainly had good times in theaters too, I like movies, but I've grown a bit tired of it. Now generally the only time I go to the theater is the live showings of The Room.