definitely pass the plate number along to the game warden. They will probably contact the driver and at least let him know that he was observed violating the law and maybe make him think twice about it, before he does it again.
Now, if you had a video cam, like I did last week, and you take a video of the people and vehicle doing the trespassing, you can send it to the game warden, and he can then right the ticket for trespassing. I got good video of the vehicle, plate number, driver and passengers faces, guys with them on the quads faces, (and the fact the quads were unlicensed). Contacted the local gamie, told him what I observed, gave him all the info, and then he got excited when I told him I had video of the whole thing. He asked me to send him the video, and he was going to contact the individual and write the trespassing tickets. I love this little Sony Handycam and plan on taking it on every trip into the woods now...or out fishing too. Great way to make sure they get cited and fined. Hard to argue against video...