Contact your local game Warden.
Buy a Trail timer camera that texts you pictures.
I use this one but there are many to chose from
http://www.trailcampro.com/hcospartangocamnoglowverizonreview.aspx

When you get a picture of a tresspasser, call the game warden and have him ticket them.

It starts out as a small fine but grows to be very expensive if they get caught a second time and the losers know it.

All you have to do is catch a couple and press charges where they know you will do it.
They will find another place to fish