A very simple script, just prevents building beyond x blocks above the floor.
Download: http://www.mediafire.com/?5jj65d2poesq2am
By altering MAX_HEIGHT at the top, it changes how many blocks above the indestructible layer the players can build.
With maps like Goon Assault or Pinpoint, setting it to '2' would stop people from building the usually dreaded 3-high walls!
Hope someone finds it useful sometime.