How to keep my neighbors who live in an apartment off my property?

I live in a small town (pop. 3600). Recently an apartment building was built 2 doors down from me. My property is "L" shaped and I own all the property directly behind the apartments. The owner of the apartments has ured me many times that he has told tenants that the property does not belong to the apartment tenants and is off limits. The words have not soaked in. I constantly caught tenants using my yard as a dog park. I recently put up two very visible "Private Property No Tresping" signs. This has not deterred them. This winter I can still see tracks in the snow of owners walking their pets in my yard and returning to the apartments. The nerve of people to feel they are so entitled to just do what they want... It really makes me mad. I am to the point now that I am afraid I am going to have to put up a fence (at my expense) to keep people out of my yard. What other alternatives do I have?