It wont do much good but did help some...first put signs on the island, trash can lid, vacs or wherever your cans are about illegal dumping. I used something to the extent of: NOTICE NO DUMPING ALLOWED No Dumping of household trash, tires, batteries, etc. Violators will be prosecuted in accordance with (whatever the county and state ordinance your in and the code section) At least if you catch someone and they give you grief about it you can point. Unless you police it it will not get better.
I had one contractor every sunday bringing 6 to 10 trucks in to clean out, they were spending good money to vac and wash each truck, but every truck had at least 1 30 gal bag full of trash. They have been very consistent on washing all their trucks weekly, i met them one sunday and we discussed this, the fact that im having all my cans filled from their trash and no room for other customers to use so they pile the loose trash up and it blows everywhere and becomes unsightly, and how it gives others the idea that they can drop their stuff off as well. Now when we go in on sunday morning i leave the dumpster unlocked for them, they dispose of their bags in the dumpster and
lock it back for me. It is a win win, im still making the money, im not dumping their trash, my cans stay empty for the most part, and my lot stays clean.
If its rogue individuals with no consistency, that is a different story and unfortunately its either gonna be part of the business or if you want to make a change you will have to put in the time to change it.