Some complaints countered + 1 legimitate complaint (IMO)
Here are some countered complaints:
1) DUUUH, build a fence around it.
Ok, well first of all, as many have already pointed out, what's any kind of fence to stop the most curious of individuals from still breaking their way into the fog? More importantly, we're talking about a demonic/cursed/evil patch of land in the forest whose boundaries are demarcated by fog. I mean, I know the main characters described it as completely motionless. But it's cursed; and just in general, what's to stop it from not moving like regular fog when induced? The fog can easily expand, thereby expanding the cursed patch of land, to engulf the fence. And especially considering the evil dwelling in this foggy region, that brings me to complaint #2 and its counter-argument...
2) Bomb/aerial attack/remote destruction/etc.
We're talking about an evil presence that dominates this foggy land, that can possess people and clearly holds some spatially disorienting powers. What's to say that this place isn't in another dimension of sorts that make the weapons completely miss the target; or, better yet, what's to say that these suggested attacks won't simply vanish/disappear/become totally ineffective after crossing into the fog? I'd say that since this is a paranormal circumstance, it calls for a paranormal means of putting an end to it.
The only real major complaint:
For something like this, the authorities really should be contacted to deal with it. Government bodies and big organizations have the best resources at hand to both keep the cursed region quarantined and off-limits from the general public and attempt to undo the curse.