Just say: No, I can't tell you why you're no good.
Simon Meth tells us how you reject a candidate.
Send rejection emails to candidates who have not had a face-to-face inteview with you or the client. Call those who have had a face-to-face.
Thank them for their time and interest. Then say: "We are not planning to move forward with you at this time."
When she asks why, say: "I'm sorry, I'm not able to provide you with any details.
If someone asks for a reason a second time, repeat your inability to give details. "Company policy, you know, old chap."
After that, 99.9% of them give up. Those who don't are insane.