Today I was cycling home from work. I was just cycling past a group of two girls when one of them hit me carelessly when she was throwing her tin can of energy drink on the ground. They said "sorry sir" in a mocking way and started laughing as I cycled past. I didn't care about that the tin can hit me. However, I felt very angry about the impact on society and the environment of their careless actions (throwing a tin can on the ground). A bit later, when we were waiting at the traffic light, I wanted to address my disapproval of their actions. However, I was not able to come up with a statement that seemed effective. They seemed so indifferent towards it, throwing their litter on the ground like they did not care.
Now, after this event, I feel bad about not addressing it. I still can't think of a statement that would make them rethink their actions, and hopefully, act differently next time.
Possibly relevant: Although I'm 22, I look rather young for my age. Although they are definitely younger, they might not see me as an authority.
So, how can I address someone's actions in such an event in a way that at least shows my disapproval, and hopefully makes them never litter again?