I use a 3' crowbar that I carry with me tucked into the frame.  As others have said, move the rock to the center, a couple good whacks, wlll get them out.  Over the years I've had a couple that I couldn't get out by pounding on it.  I then heat the heck out of it with a rosebud tip then hose it down with cold water.  They crumble and fall out on their own.