To talk to them or not, to state the obvious, is your decision. I have had problems with neighbors over the decades. Talking some times works. Well, no, it has never worked, well it did once.

ONCE and that was because the trespasser was a kid with responsible parents. Talking to drug dealers or drunks is a waste of time.
I have chased off trespasser with guns shooting on my land. I would NOT recommend doing what I did to most people. Supposedly I have the training and experience in such matters but it still is not a good situation.
I have a "problem" property line. Years ago the owner of the land next to us did not have a clue as the property line. Why? Because he tore down the flagging.

Why, I have no idea. The house he built is not square and plumb on the lot because he tore down the flagging. Their septic field ran 10 feet along side the property line. I knew the guy would put the septic field on our land which would be a mess.
I got the survey company out and they flagged the line, put in a pin between the corners along with some other work I needed done. It cost me a few hundred dollars but that was cheap compared to lawyer fees if the septic field had crossed the line.
When the flagging showed up I got a call from the property owner ASAP asking why the flagging was on their land. My answer did not make them happy but problem solved.
Find out the posting laws in your state. NC requires that the sign be a certain size, be placed at given distances, and now we can use purple paint which is really nice. Once the surveyor flags the line put up the signs per the law.
If they continue to block your trail at a given point, clear the obstruction, and setup a hidden trail camera...
Talking to them? You suspect they are stealing. They have trespassed repeatedly and have blocked your access to your land. Them thar actions ain't friendly. Talking on the phone maybe. In person? On their land? I would think about it long and hard.
Go read the thread from MDBARB. It has to be one of the longest running threads on TBN. It is about an easement dispute between him and a neighbor. He lost. He lost lots of money. My impression, is that MDBARB might have lost some court brownie points during the lawsuit because of his face to face interactions with the neighbors.
Later,
Dan