I'd say install a hidden camera to catch your neighbors in the act if they try dumping their trash on your property again. If they do it before your court date, you have evidence against them.

Time to move. They will never change their habits. It only gets worse from here.

Paint your own wall/fence with some obscenities and blame them for vandalism again.

They sound smart. You, not so much. The only way to get even is install a camera and catch them in the act of throwing trash in your property.

I know this comment is too late, but they knew what they were doing, so I wouldn't have thought any good would have come of asking to them stop, only that they'd cover their ass in the event of you taking action. That is, unless you'd already installed cameras and gotten evidence of their behaviour and can get evidence of any possible retaliation, or had asked other neighbours (whom you trust) if they would back you up. I guess you've probably figured this out by now, but if not then this might cone in handy in the future. Good luck with court.

If you're gonna get arrested, you can't get arrested for things you didn't do! This is an opportunity to remove the issue of your neighbors without affecting your future.

Here's a possibly unorthodox evidence idea- print out your bank statement/reports where it says what you bought, and point out trash from businesses you do not use. If you shake the charges, you can probably get some kind of legal retribution at your neighbors, but whether you do or don't, they're gonna target you from now on because you had the "nerve" to call them out on their utter and ridiculous lack of basic neighbor- or even human- decency.

So how did it go?

why didn't you do that in the first place?