Corlia,
I usually try to hide the fact that an interview is failing miserably. A savvy candidate can tell, however, by the fact that the interviewer stops asking probing questions and seems in a rush to finish.
If it's a "good candidate" but not fit for the job, I try to give them subtle advice. For example, we were interviewing a candidate who only had experience volunteering in small libraries, unfortunately she was applying for a paid position in a large library, so near the end of the interview I asked if she had ever considered getting experience by volunteering in a larger library.
Another candidate was totally out of his league so the last question I asked was "You are in competition with people who have a lot more experience, tell me what you feel would give you an advantage in being selected for the position." I don't want to dash their hopes, but let them know gently that they may be lacking in the experience pool.