When I was 14 I tossed a piece of packaging for the chips I was eating on the ground. I don't know why I did that, I'd been so against it as a good little kid, I think my mind was just experimenting at the time with whether I really needed to give a shit about this anymore. Probably some kind of "edginess" I was cultivating perhaps. Anyway, some middle aged teacherly guy picked it up in front of me and put it in the bin. Then he gave me a statistic about how our city was the "nth cleanest in the world and we should keep it that way". I was by myself but kinda scoffingly shrugged it off as he walked away to show I didn't care what he thought. But being called out like that and feeling that hot flush of angry embarrassment and being forced to pay specific attention to my actions instantly and dramatically recalibrated that drift in my values on the issue of of littering in a permanent way. It wasn't because they made an especially good point, in fact I didn't find the statistic particularly compelling I mean of all the reasons to do the bare minimum of decency that seems like one of the worst, like it's some sort of competition or something. Nevertheless it was just a reminder at the perfect moment that no, this isn't going to be acceptable even if there's no obvious consequence and you shouldn't start to feel okay about this.

The fact that the guy was kinda lame and had such middle aged dad and teacher vibes about him I think made all the difference, there wasn't an angry confrontation, but it was still firm. He backed off and walked away straight after he said his piece rather than giving me the chance to turn it in to an argument where I might feel rebellious and victorious about it, he just calmly left me to stew in the fact that whatever bravado I might have put on for him, he didn't care and I was going to have to reckon with why I ever thought this was going to be a good habit to start.

I bring this up because maybe if you have the opportunity to you actually should say something, though obviously carefully and not too aggressively. Sometimes it makes a difference even if by their response the person would appear to indicate that it didn't.