Xref: news7 alt.home.repair:314535 rec.gardens:213147

you need to get a hold of the armature in a way that wont damage it so you
can unthread the impeller. some units have a slot at the top end of the
armature for a screwdriver or the top end of the armature will stick out
enough that you can use a soft jaw vise to hold it while you turn the
impeller off.