It's tricky, we know. Because in the past we've suggested that what viewers and voters don't want is for the debate moderator to insert himself/herself too much into the proceedings; that people don't tune in to watch them.
That said, moderators ought to fee free to step in, politely, and redirect candidates when they so blatantly ignore the stated question. Palin just did that when Gwen Ifill asked her what her true Achilles heel was. (When it was his turn, Biden at least used the basic framework of the question to form his response.)
The debates are already so tightly scripted in terms of the ticking clock, why waste time by having candidates give answers to questions that were never asked?