Land

Here's a nice walkthrough on how to adjust the settings on your land. The land can be a no-fly zone.
http://blog.coveislands.com/2009/03/17/the-about-land-panel/

This link is about a group owning land:
http://blog.coveislands.com/2009/03/31/group-owned-land-in-second-life%E2%84%A2/

I haven't found anything on building invisible walls in SL, yet.

Actually, owning land in SL is pretty weird. You have the ability to own land, but you have to pay a land maintenance fee each month.
https://support.secondlife.com/ics/support/default.asp?deptID=4417&task=knowledge&questionID=5198