I only freelance so I don't have to do whiteboard interviews but if I had to I would simply not do it.
"I'm not a recent graduate, I have over three decades of experience in development roles, some of them lasting over four years. I didn't keep those jobs by not being able to code."
Well we believe in putting all candidates through the same process and ..
"I don't want to work for you"
OK, maybe not, but more like:
"Do you have any training in interviewing? I don't think you do, because two of those innocent opening questions you asked me to establish some rapport were illegal and I could sue you for asking them."
"So I am going to drive this interview since I DO know interviewing and I know better than you how to demonstrate why you should hire me."
Whiteboard interviewing is bullshit. Refuse it.