This add-on creates an invisible box around the spawn.
Players cannot leave or build outside of the box.
You can move and resize the box using /setbox [SIZEX] [SIZEY] [SIZEZ] [POSX] [POSY] [POSZ]
Size is box radius in 2x cubes.
Position is the center of the box.
If you want the box to sit on the ground, set POSZ to the same number as SIZEZ.
The walls of the box are very strong.
If someone does leave they will be killed.
Vehicles will be respawned if they get out.
I have no interest in modifying this add-on any further.
Download:
http://www.mediafire.com/file/zjc2s73vxymy7b9/Server_Box.zip