I would mail them a politely written registered letter telling them to remove their stuff within a week.  Then I would have a survey done to verify the property line, and build a nice looking white plastic fence or something similar that would be the same as I would want next to my house, including a gate at the road.  I have found my stress level and ability to get along with neighbors is much better when I know that I took the high road and bent over backwards to try and get along.  If that doesn't work, then involve the sheriff and attorneys.