Craftsman 12-gal wet/dry vac...

Hey all, I'm trying to use this craftsman 12-gal wet/dry vac to vacuum up debris from sanding drywall compound, and it has this 'blower' feature. The problem is, there seems to be no valve or switch to close off the blower, and when I suck up all the crap, the vac just blows it all out the blower, filling the room with drywall dust! I've read the manual, and done all that I can think of, but can't seem to figure this out...Is the drywall dust simply too fine for a vacuum with this power? And in case you are wondering, I have the filter in.