I'm not saying it's right to toss your trash into your neighbors yard, but if you're not allowed to use a trash bin, and nobody ever comes to your house to collect garbage it's not not hard to imagine why it happens that some folks will chuck their trash anywhere.

Most people only do those types of things because they've been backed into a corner and don't know what else to do. Give folks an alternate that lets them sign up for stuff without hurting themselves in the process and they'll do it. We've had some great services pop-up to help (like temporary email services and bugmenot) but as those become less effective people will do whatever is left to them.