Wasn't directed at you.
Several have given responses saying maybe the neighbor was mistaken or didn't know the boundaries.
OP stated land was clearly marked.
I did not make any suggestions to action - harsh or reasonable.
Just stated that based on the 1st post, the neighbors are knowingly trespassing.
I would talk 1st, but based on my experience with people knowingly trespassing, it will not be enough
and further action will be required.