Many, many years ago, when I was in a pram in the garden, my Mum used to put netting over it to stop the magpies pecking my eyes out - it must have worked 'cos I've still got two eyes...
You could try hanging up some CDs around the garden, magpies don't like reflections and I've heard it said painting eyes on things around the garden works too as magpies don't like being looked at - no, I haven't made it up!