Is there any way/workaround for locking layers?

Now and then I get DWG-drawings where I need to add some lines and small 2D blocks. As I want to set dimensions relative to existing geometry, I need to lock the existing (imported) geometry. In the layers dialog I can control visibility, and to my surprise there is no way to lock layers.

Any suggestions for a workaround is appreciated.

/Magnus

SolidworksDrawings And Detailing