I had a problem with a neighbor about a different situation but like you they were doing things that were not legal. One call about what they were doing followed by a a visit from the authorities I never had the problem again and surprisingly they were very nice to me after that.
Make sure your property line is posted according to regulations so that there is no doubt that he is knowingly trespassing. Yea I know he knows but if you catch him doing it again you want it to be so solid he has no way out on any technicalities.
In addition it might be worth a call to your game commission. In PA it is illegal to harass anyone who is legally hunting. Not sure if pouring that stuff on the ground fits that category but I think that the more you impress on him that you are not going to take it the sooner he decides to give up.
Hell I would even check with the local environmental dept if you catch him spilling stuff on your property. They might be obligated to checking out dumping of an unknown liquid on your property. The cost to determine if it is hazardous or not and any removal they may require would be placed on him. 20 years in that business and I can tell you that it would not be cheap.
I think I would shoot at any game that was near by every time I saw him anywhere near the shed that you hunt out of.