My work-around is to place an image tile with transparent background and no image link on top of tile I want to be read-only.
Note that the Aquarium Canister Filter is grayed out some because I didn't make the image tile 100% transparent to remind me that the image tile is on top but there is no reason you couldn't make it 100% transparent.
If you want everything on dashboard read-only, put a transparent image tile over everything.
