Making candidates white board is basically an ego driven hazing process, "we had to do this stupid exercise, so you have to as well! Ha ha!"

It also gives people a bad taste in their mouth, don't ever forget the candidate is interviewing the company as well.

The best response is to immediately walk up and leave when asked to white board something, stating something to the effect of "sorry but if this is your company culture, I'm not a good fit". I know several people who have done this and were hired anyway.

Look at a candidates successes and body of work, if their competency isn't obvious from what they have demonstrably accomplished, they probably aren't the right candidate. No amount of white boarding (or the nonsensical "thought exercise" questions) is going to change that.