MaterialApplyPolicy component as seen in the Scene Inspector
The MaterialApplyPolicy component allows for controlling the ability for the Material Tool to apply materials to the slot hierarchy's SkinnedMeshRenderer/MeshRenderer components.
Setting this to false will prevent the Material Tool from applying materials to any SkinnedMeshRenderer/MeshRenderer components in the slot or child slots.
On an avatar, you can try adding this component to the root of the avatar, making sure the avatar has a Component:ObjectRoot, and set CanApply to false. Doing this will ensure that the avatar's material cannot be changed by the Material Tool.
Adding this component to the Slot of a MeshRender and setting CanApply to true can allow one to explicitly change the material in specific places on small objects.