iron golems spawning inside unspawnable blocks

spent many hours yesterday trying to prevent golems from spawning in this location, on the outside of a villager farm. they are spawning on the tops of the posts, inside the stair blocks. i replaced everything under the roof with a layer of glass blocks, and they continued to spawn inside both the glass blocks and the stair blocks. i replaced the glass blocks with lower half slabs, but they continue to spawn there.