It starts small. It always starts small. Suggest reading some Arendt.

Besides, littering isn't necessarily small potatoes around here. There are signs at the roadside that warn of potential fines exceeding $6000.

Is he littering on the public roadway where those fines apply? Because if he is, you probably don’t have any defensible private interest at all.

If he isn’t, the sign is irrelevant.

Hard to say, that'd be something to ask a local attorney. Which you can bet I'd do, if people were illegally dumping trash on my property.

No, I'm saying that dumping a ton of rubbish outside someone's house is not something that a well adjusted person does regardless of whether someone tells them to do it.

But if you must - it's even less of a defense now, because in WW2 Germany there was a significant amount of social pressure to fall in line. In the modern day no-one cares if you just refuse, or get a different job.