I am not sure of a good way to lock down a layer, but one way to do it is to "edit sheet format" by right clicking in the drawing area and putting your reference on your edited sheet. I am not totally sure of your application, but if you do this, once you are out of "edit sheet format" you will not be able to select the geometry below.
Another way, which is probably slower, is to fix the geometry that you are trying to trace and just right click and use the "select other" tool to choose your selection each time.
Hope this helps.
Thank you very much. The editing sheet format was a great idea and it works perfectly.