Yes, the right move is definitely to assault a person who's doing the job he was hired to do.
Gently putting your hand on someone's shoulder is assault if it's unwanted. Water is less pleasant.
Here's some people on a random website claiming to be lawyers, but also saying the right words: that they can't give specific legal advice and that the OP should not do that and/or get a lawyer. https://www.avvo.com/legal-answers/can-i-be-charged-or-arres...
There's an article from San Francisco about a man being charged after hosing down a homeless woman. Ultimately the charges were dropped, but the answer to your question is yes with caveats.
Besides, littering isn't necessarily small potatoes around here. There are signs at the roadside that warn of potential fines exceeding $6000.
If he isn’t, the sign is irrelevant.
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.
But also, it's a terrible analogy because a punch to the face is potentially lethal force but delivering a newspaper isn't. Which, obviously, matters.
Spraying someone with a hose in this situation is battery. Unless the newspaper is being launched at your person with a potato gun, you're going to have a hard time coming up with a defense against that battery charge.
Unlikely, but not worth it. Also, MUCH more importantly, wrong target. Don't go after the laborer. Go after the capitalist paying him to harass you. Go after the owner's wallet and time, not the delivery guy's clothing.