Sometimes, the only way to get the negative aspects of an idea across to someone with a bright look in their eye is to stand back and let them do it.

Let them fail.

And then, when they run face-first into that brick wall (exactly as predicted), don't point and laugh and say "I told you so!"

Instead, help them to their feet and to get dusted off. You can then share stories of your mutual, independent failures over a beer or something.

(And the next time? You still don't have to tell them that the idea is a dead-end; if they're smart, they'll ask you questions before they burst out in a sprint.)