ShapeLocker is good but may not be exactly what you need because even when shapes are locked and cannot be deleted/changed they can still be hidden and replaced. If you have any other AddIns you might also find that they can crash when the other add in tries to select or modify locked shapes.
Here's an example of a slide for you to test with a locked shape on the large slide master (I didn't use shapelocker but the method is exactly the same.)
Sample
If you select the large master and open the selection pane you will see it can be hidden easily.