There's no surefire way to stop deer from eating anything. But, there are a few things you can try. Get a deer/rabbit repellent from a garden center and thoroughly spray your plant with it. I have used one that contained putrefied egg solids and worked well. Respray as needed. You could also try putting up a fence, although that is more expensive and unsightly and deer may jump it. Either way, unless the deer are eating the hostas down to the ground, the plant should be able to grow new leaves and rejuvenate itself.