Allen always has the best ideas
Not only are Allen's ideas effective, but they are a reflection of a good person.
As far as getting lawyers involved, I would steer clear of doing this. You need to diffuse the situation, not escalate it. White trash are not afraid of lawyers and police. The justice system is a joke. If the situation escalates, you have far more to lose than they do.
I would first attempt Allens idea, my backup plan would be to press upon them, how it is potentially dangerous for them to tresspass on your property. They obviously don't respect the sanctity of your land, and only you can fix this. It's is always better to try the nice way first, but there comes a time when the strategy of being nice is no longer appropriate. If they are tresspassing on your property, the police are going to lend you much more credability than they will a couple of punks. In most states you can use physical force to defend property. Not deadly force, but physical force.
As for your former employee, you can try to speak to him. I would explain that it looks like sour grapes on his part, and people will see through it once you explain that he is a disgruntled former employee. I would also explain that future employers monitor facebook pages and do google searches of them and that it does not look good when you are a drama queen that likes to stir up trouble for past employers. I would explain that he has nothing to gain by doing this, and I would point out the potential embarrasment that will come when you start telling your side of the story.