Some of these stories about super experienced ("10+ years") developers not being able to answer the most basic of basic syntax or "algorithm" questions seem far-fetched to me.
Really I don't think those sort of on-the-spot tests/trivia questions are representative by themselves. They may be a useful part of a larger investigation.
Two things to stop code sample plagiarism: choose a unique code challenge and, if you're really worried about it, give the client a laptop and tell them to bang something simple out right there and come back in 30-60 minutes to check.
Honestly, though, I've rarely depended on either code samples or code trivia to determine if someone is a good hire. If you sit and talk about dev with a person, you can tell if they're on their game or not 90% of the time. The sample/on-the-spot tests can help weed out that extra 10%.
I'm not sure how this is possible either, but the world works in mysterious ways.