What is a nice way to end an interview that is clearly going badly?

As an interviewer, I occasionally conduct interviews that become painful as time goes on because the candidate is doing so poorly. I have the impression that, in these cases, the candidate internally knows they are not getting the job, and would just like to end things as soon as possible (as would I).

In the past, I have handled phone interviews of this type by ending a little early and giving a standard closing. However, I have empathy for the candidates and would feel better if I could say something nice without being dishonest. They’re not getting the job, but I may still respect them and honestly wish them well. I’m not really sure how I could tactfully express thoughts like this, though.

My question is mainly about phone-based interviews, but I’m interested in answers that also apply to in-person interviews. To be clear, this question is how, specifically, to be nice at the end of a bad interview, so I’m looking for something more specific than simply ask how to end a bad interview. (Hence I don’t consider this a duplicate of questions asking how to end a bad interview.)