My solution was to build a wooden barricade out of simple 2 x 4's with one brace on it. The storebought sign which I screwed and glued to it says "STOP" "Private Driveway - Do Not Enter". I put a chain in concrete on both ends of it and padlocked the barricade down to the short chains so you cannot move the barricade out of the way unless you tear up the thing which is at least a misdemeanor, etc..
The ends are sort of like an A with a long brace leg running across the bottom 3' feet long. One diagonal brace across to the center holds it steady. 1/2" plywood on the center nailed and screwed in with sign on it using construction adhesive, Liquid Nails so it cannot be removed. Some screws holding sign are only 1/2" where they are only on plywood. Used 1 & 1/4" truss head screws where there is plywood and the 12' x2x4. They can't get the sign off but might be able to destroy it.