You can use the filter panel. Set the first column as keywords and select the top-level keyword. Expand that keyword's hierarchy (there's a little tick in the column's mini menu. Then select all the child keywords. Now select all the visible images - ie those which have child keywords assigned - and click the parent keyword again. This now means all those child keyword images are selected, so choose Edit > Invert Selection. And you're done.
PS round here we insist on signing posts with our real names - see http://thedambook.com/smf/sign_your_post.htm