I've written about this before, but following on from yesterday's job post, it's still relevant.
When I posted the job description, I embedded, in the middle of a long paragraph - EXTRA: What's the capital of Peru?. Any respondents who did not include the answer to that question in their cover letter were instantly rejected.
This comes from van Halen. In the 80s there was a rumour doing the rounds that van Halen would ask for a bowl of M&Ms backstage, but would refuse to play if they found any brown ones in there. Everyone regarded this as yet another example of rock star excess. However, there was a good reason for it. Their lighting rig was big and complex and one mistake in setting it up could result in collapse, and injury or death. So, alongside the complex technical information, their manager included the clause about "no brown M&Ms". If the venue manager did not question this unusual request, van Halen knew that they had not read the contract properly, the lighting rig would probably be unsafe, and hence the gig was off.
Take action: Take steps to qualify people, whether job applicants or potential customers, before they take up your valuable time.