There is no perimeter fence by the developer? Is your lot their nearest access to the main road? Was there any written agreement that a portion of your lot is to be used by others?
A right of way is not free. If a right of way is granted to them by the court, they will have to pay you for it. I am assuming that both sides will not agree anyway.
Get a fencing permit. One of the requirements is a barangay clearance, where you will mention to the barangay that there is an unauthorized use of your lot. They will either hold the clearance or give you a clearance. If you do get a clearance, then getting the fencing permit will proceed smoothly. Once you start construction, the only way you may be asked to stop it is by a court order, which the other lot owners will initiate.
If your lot is their only way to the highway, it will take some time before you can close it off though. If they have access to other roads, then it shouldn't be a problem.