If you paid them for the time they spent on the project surely you'd own it? The actual issue seems to be if they've displayed they're more competent than your entire team why would they want to work there?
1) Because I'm sure they'd be excited to implement their idea, and see it succeed, and 2) because they'd be given more responsibility and power than they'd find elsewhere.
I wouldn't assume that an expert in their field, someone with the expertise to potentially achieve global recognition, would care about a half-assed company not handling their security properly. More responsibility? They probably want less. They don't need or want anymore power than they already achieved with their own mind. That's how introverts roll.