If someone solved a significant problem at your request, but despite this you did not hire them or otherwise compensate them for that work, I don't see how you have any right either legally or ethically to expect them not to then use the work to advertise what they can do to anyone else. The premise is one-sided enough already (though the argument elsewhere in this HN discussion that it might be preferred by nervous interviewees is an interesting one I haven't noticed before) and I think expecting any sort of loyalty or confidentiality after the process has ended is rather optimistic.