RE: Someone else hunting in my stand...
As you mentioned it is your Dad's property and maintaining a healthy / neighborly relationship is also a good thing...
It sucks for you, no doubt, but consider taking the permanent stand down and trim the tree for a climber.
You can leave it attached to the tree with a lock and use it when your around. At least it's not a comfortable "free" seat for your Dad's neighbor ~ correct?
I know it's a little more work / money and may affect this years hunting.
But if you have more pressure from an unwanted trespasser your already battling something anyways.