I should preface this with I have no experience with European hornets (and
that's usually where I decide to shut up and not say anything at all), but
have you tried an entrance reducer with individual bee sized holes drilled
in it?  It wouldn't keep them from killing bees outside the nest, but might
keep them from getting inside without restricting traffic too much.  That
is of course assuming you can make a reducer that will allow drones to
escape without letting hornets in.  I couldn't find an exact size on them.
Just a thought.

Jeremy
West Michigan

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