The Universal World Owner's Manual

Walls can be placed along the edges of your map squares, and can be a useful, quick way of fencing off areas of your world, without having to worry about generating loads of wall models.

The in-game 'World Editor' (F8) can be used to add walls to your landscape.

Stand near the beginning of a wall and use the command *removewalls. The server will confirm the number of walls removed (you will need to relog to see the change.)