It is quit easy
When you use Polyworks then pick a color you dont use somewhere else and make the walls you dont want others to see when your map is ready, after making the walls you can change the polys from normal to only player collides or to what you want to heave and set the Opacity to 0%, after setting the opacity press enter:
from now on nobody can see the walls, even you can see them just if you click on them in polyworks or turn on the wireframe
Btw: for getting all those polys once: press Ctrl+B for marking all polys of the picked color
