I usually will call out loudly to someone throwing trash on the floor and let them know they dropped something. With the exception of one dude, everyone immediately picks it up out of embarrassment and throws it away. The one dude, in reference, looked at it and said, "That's not mine", to which I said I saw him throw it and pointed to the nearest trash can and said he needed to pick it up and throw it there, he eventually did and we both moved on with our lives, otherwise I would have been forced to use the "Your mommy isn't here to clean up after you" card.