Can I save my fortress with a wall?

(Move to the TL;DR section if you don't care about the context.)

So I was digging too deep when I found some giant cave spider. I think I was bothering it, since it proceeded to decimate my fledgling military. Once I had managed to calm it down, er, permanently, the tunnel revealed another welcome guest; a forgotten beast. This monster, which apparently took the appearance of a giant grasshopper made of iron and with five or so mouths, then tore right through a door blocking off the mines in my fortress from the rest of the place.

Now, what remains of my military is fighting the beast, but I am sensing their inevitable failure. Thus, I need to enact Operation: Grasshopper Blocker, which consists of me building a wall. However, remembering that the grasshopper could destroy doors, and knowing that giant cave spiders (which I'm sure there are more down there) are building destroyers, I am uncertain whether or not my wall would be enough.

TL;DR Are constructed walls (wooden and other) safe from building destroyers?

Followup question: is it possible to cause a cave-in that will permanently close a passage?

Yes, constructed walls are safe from building destroyers, as walls are a type of construction rather than a type of building.

From the Building Destroyers page of the DF wiki:

Constructions (walls, staircase, floors, etc.) are still safe, since they're processed the same way natural terrain is for most situations.

Rave turned answered the primary question. In answer to the follow up, yes a cave-in can permanently seal a tunnel.