Have you talked to this neighbor? Saying please don't put this in my yard. If you do not want to have the confrontation a call to your local law enforcement folks and explain to them...then they can contact the neighbor. In either case thy will know it is coming from you.
I you removed the debris while they are "watching" they may also get the hint without you having to play the upset neighbor card.
I agree with KMS on this. First talk to them and ask them to stop. If that does not work or you just feel uncomfortable about speaking with them on this subject, call your local PD or Mayors office for a suggestion from them on who to contact. While you may own a large piece of property, your neighbor may think its just an abandoned lot or township owned.
Have you thought of putting up signs that say no dumping allowed? That alone may stop them. Anyway it will give more power to the PD if the property is marked in such a way and they continue to do that.
If it is in a subdivision with an HOA, contact the grounds enforcement person. If not, you would need to contact the local police authority on the non emergency number.
But........before all that ....maybe you can resolve it with some diplomacy?
What are they putting in your yard? Maybe you can suggest an alternative solution for them if you are on speaking terms at this point? It really depends on your relationship and how long you have been neighbors and if they just started doing this or it has been an on going issue?