If you want to do it in Python you can execute the following:

 toRemove = [block for block in bpy.data.materials if block.users == 0] for block in toRemove: bpy.data.materials.remove(block) 

If you want to do it using Blender's Python API you can use the following. Just make sure you unlink the unwanted materials before doing this:

 toRemove = [block for block in bpy.data.materials if block.users == 0] for block in toRemove: bpy.data.materials.remove(block) 

If you want to do it in Python you can execute the following:

 toRemove = [block for block in self.bpy.data.materials if block.users == 0] for block in toRemove: self.bpy.data.materials.remove(block) 

If you want to do it in Python you can execute the following:

 toRemove = [block for block in bpy.data.materials if block.users == 0] for block in toRemove: bpy.data.materials.remove(block)