I would rather not get the job than have a future employer risk my current employment by going against my wishes..If this is what you do then it shows you have no respect for your employee's.Really? So references should only ever be taken from the selective few that the interviewee thinks will say something glowing? If the reference given is somebody not known to the interviewer, what's to stop the candidate giving his best mate's number (which I know has been done plenty of times).
When we've been looking to employ anyone, I want to ask as many people as possible. Someone specifically saying 'don't ask so and so' would certainly set alarm bells ringing with me. Alarm bells in an interviewer isn't a good thing.