This might be a bug. If a layer is locked or guides are locked I can't delete all of them. Say I use the Layers>Create Guides and I select remove existing guides, nothing will happen if guides are locked or the layer they are on is locked. Same with the View> Guides and Grids> Delete All Guides on Spread. I can see why this would happen as you might not want to delete every guide sometimes but it contradicts what the option says and becomes annoying.