So you want an area to be visible but you dont want peeps getting there? or you want a invisible wall? could the invisible man walk through an invisible wall?

well anyway, just select the aaa texture and make a block where you want to block people from getting to. now select tie to entity, and then select func_wall, now in the renderfx, select solid, and in the x amount leave that at 0. I think this blocking method is best for preventing players from reaching somewhere they dont want to be going, like on a roof for instance. Not blocking certain places can make your map unplayable and spoiled, because there are always players wanting to get to interesting places, trying to exploit your map :P so dont let your hard work go to waste, and block um.

Now you have your invisible wall, watch out for the invisible man, or should I say hollow man?

*UPDATE* you can also make a block and apply the clip texture to it and this will block player movement, but not bullets or grenades(you do not have to make it an entity it will show up in map invisible).If your map is big and running short on clipnodes this could also help some.