When in the Layer Settings dialogue box (Ctrl+L PC, or cmd+L for Mac) you can Lock layers off within a layer combination once items have been placed, i.e Roof Slabs, Floor Slabs, Meshes etc, this is to ensure they aren’t accidentally deleted or moved while working on your project.
They can be easily unlocked temporarily to modify them by selecting the Padlock or clicking on Unlock Layer from Layers within your Right mouse click menu whilst working on your project.