[SC] My Elderly Neighbor is Destroying our Property

I live in SC, USA and own 5 acres of land. One property line was originally made to run down the center of a natural creek, my neighbor owns the 50 acres of property on the other side of the creek. He is 86 years old and when we built our house in 2019 seemed friendly.

Now this year we have discovered he isn't but we were not aware before buying this property that this man had a literal 50-year-old feud with my husband's grandfather. We only found out about it when he told us about it this year after my husband caught him spraying weed killer on my vegetable garden when he thought we weren't home.

This man is the reason why my gardens haven't grown since moving there, several of our beautiful old oak trees have also mysteriously died for no reason, and we no longer feel safe to own pets because they go missing mysteriously. None of this is stuff we can prove, but we have now discovered on top of this that he is sabotaging our creek bank while reinforcing his own in order to shift the creek onto our property.

Our creek bank, which held just fine for decades before we bought it, now has major erosion issues and we have huge collapses when it so much as drizzles outside. One portion of the bank collapsed and totaled our tractor when it was dragged it down with the dirt down into the creek which is 5 feet deep in this area. My husband took the extreme measure of putting on camo and sitting in his deer stand after dark to see if he could catch the neighbor and sure enough, he caught this man with his sons and grandkids shoveling dirt from our creek banks, removing rocks, etc and using them to reinforce their side of the bank. They have even stolen rocks we brought in to fill in the big collapse by taking their tractor onto our property and taking the rocks while we were at work then using them on their side of the creek bank.

We're going to try and get video evidence of them doing this but in the meantime, we're trying to figure out if the property line actually moves with the creek. The spiteful side of me is hoping it doesn't so we can just wait for a while longer, save our money to get it surveyed, then let him drown in karma when we are able to fence the creek off from him since it has moved almost completely inside our original property line thanks to this neighbors tampering.

But if it does move with the creek then we are in a bind. We're drowning in medical bills right now due to our baby's NICU stay when he was born and I had to have emergency surgery in Feb. We can't afford to have a grader come in and fix all of this damage and we both work full time so he has 40-60 hours a week of us not being home to do what he wants. We set up game cameras and they go missing, we can't afford to buy more. We also can't afford to move.

Any advice is appreciated, thank you.