Welcome to Beesource Dale! Do some searches for "Hornet" and see what you can find here. (There are a few threads.) If you don't find anything that works, ask a question in the forum. From what I read just now, they can sure do damage to hives.
Entrance reducers as suggested above can certainly keep hornets out of the hive (assuming your hornets are larger than bees). Entrance reducers come in a variety of forms. All honeybees can pass through 1/4" hardware cloth, but bigger hornets won't be able to.
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