@rwalia you aren’t able to delete those prims since the source USD (lab_copy) is referenced as a composition arc. by deactivating the prims, you are effectively removing those prims from your current stage without breaking the ancestral prim/hierarchy in your source USD. if those prims need to be removed for good any time you reference the source USD, it’s best to open up the source USD and delete those prims directly.
another non-destructive way to do this AND keeping your stage not cluttered with these deactivated prims is using the filter to hide those inactive prims (by unticking Show Inactive Prims). This will leave the source USD intact while keeping the stage visually optimized: