I uninstalled my wacom driver (I'd rather play PZ than use my tablet, as I'm not in the middle of a work project right now) and building walls works, as far as I can tell. For now.
It seems to be completely ignoring my external mouse.
I hope a fix comes out soon for the people who can't uninstall their drivers (keyboard o_o), and thanks everyone for the quick and helpful replies. :3
Kerilynne's post in Building won't work? was marked as the answer