In my opinion, they drew first blood when they not only came on your land but cut your trees.

I'd be tempted to hang a big blue tarp from a clothes line so that it extends 20 feet in the air and blocks their view on to your land. On the side that faces them, tie on a no tresspassing sign.

It would be one thing if they just hung a stand facing on to your property, but if you are positive that they cut your trees, that is pushing the limits as far as I'm concerned.