As a senior software developer (based in the UK), my manager has me conduct interviews for junior developers alongside either himself or another senior developer. For the most part, he gives us a free rein to ask whatever questions we want to help judge a candidate's skills and personality. He has however provided a few questions for us that if the candidate gives us a "wrong answer", we cannot recommend them for a second interview, even if their strengths in other areas vastly outweigh these "wrong answers". This is the manager's decision and for now, I have to assume I cannot change his mind on being flexible with this process.
Some of these are obvious and can be answered from CVs or cover letters, such as "Are you planning any really long trips abroad in the next few months?". Others I personally feel should not instantly disqualify a candidate, such as "Do you write any blogs or do programming outside of work?", but that's another topic. Regardless of how well they impress us in other aspects, if they give a single answer the manager won't like, we aren't allowed to proceed them to the next stage. We have to just continue the interview as normal and inform them later that we have decided not to continue with their application.
Many candidates have to make special preparations to come meet us, such as sneaking away during a long lunch break or getting out of the office early. As I have been in their situation many times before, I would feel guilty about taking up more of their time than necessary, as well as leaving them waiting for a decision that has effectively already been made. On the other hand, I fear some may not react well to being told this so quickly.
My question is: would it be unprofessional to inform a candidate during the interview that we won't be able to give them further consideration?
I should note that while I get on well with my manager 99% of the time, he is not the kind of person who would respond well to me asking "If they get one question 'wrong', should we reject them on the spot?".