To remove s.t. by slapping, banging, brushing off (as when slapping a ball out of s.o.'s hand, banging an eraser against s.t. to remove the built up chalk, brushing a bug off of one's arm).
Both the -an and -un imp. focus on the object being removed. The difference is when using -an you are affecting s.t. and absolutely have no use for the thing being removed (as when slapping or brushing a bug off of one's arm).
Slap his ball so you can grab it.
Brush off the centipede on his arm.