There comes a time when a fence makes for a good neighbor, especially when property rights of the individual owner is not being respected. You as the property owner have the right to deny entry to any individual that has no contractual interest in the property you own.
At the first sign of a problem you should try to resolve the issue peacefully with gentle reminders to let the person know that they are not allowed on the property. Sometimes you may have to go further. When an individual still decides to enter your property after you gave them gentle reminder, they are now considered trespassing and you could file criminal charges.
Another option is to invest in a stake survey to have the boundaries drawn to clearly show where one property begins and the other ends. Invest in a fence.
Do not resort to criminal activity to remove the trespasser from your property. You will be in trouble with the law as well as your neighbor.
Right now I am aware of one case where the tenants of a property that only has street parking, have decided to use the backyard of another property that does not belong to the same owner. These tenants were not give any authorization by the adjoining property owner to utilize the property as their private parking spot.
For the last several days the owner reminded the tenants of the other property that the property they are using is private property and the only method to park their vehicle is on the street. He asked them to please not park their vehicle on this private property.
After looking at the lot line, this owner is now contemplating installing a fence to cut off the easy access to the illegal parking spot and to ensure that his tenants have a place to park for their vehicles. A legal action against the trespassers is still an option that has not been excluded in order to solve the problem.