How do I make a wall that NPCs can't pass, but Heros can?

There is no way to make walls that selectively allow heroes to pass while blocking NPCs, but there are ways to fake the same effect.

The easiest method is to make an NPC with no picture. Change the Activation type of this invisible NPC to be "Step On", but do not actually choose a text-box to be displayed. This means that the hero can step on the invisible NPC, but the NPCs will not be able to.

You can place several copies of this invisible NPC on the map, forming a wall that the hero can pass but the NPCs cannot.