YOU CAN DEFEND AGAINST THIS!:
You have to mine out under every eastern wall you have, I don't know if zig-zag is enough you may have to dig out the whole area!
This is the only way, signposts won't work. Quickly
Sorry if this is too big or something but I figure it might warrant it. Mods feel free to shrink it.
Edit: Added Eastern because you only exit west when exiting a mine.