How would I make a lua wall.

How would I put up a barrier where no one can walk through it, so I can block off certain parts of a map? Is it possible?

Take a look at my reply to this post.

http://forum.facepunch.com/showthread.php?t=1487338

Easiest, limited work:
Spawn a prop that’s the right size, find a material that makes it invisible, then make it persistent.
If you want to allow certain teams through you can turn the prop into an entity, and throw a check in there and such.