A: Short answer: There’s no solution. And as a friend reminds me, if there’s no solution, there’s no problem.
Long answer: If a potential employer steals your ideas without hiring or paying you, you really no have recourse. Sure, you can inform your network. You can write about your experience on Glassdoor and/or LinkedIn. You can even notify the company’s C.E.O. or head of H.R.
But, ultimately, there’s no way (which I know of) for a potential employee to safeguard his intellectual property when interviewing. Instead, you have to know, going into the interview, how widely you’re willing to open your kimono.
Addendum (11/5/2022): It turns out there is a solution! As columnist Allison Green points out, “If you’re concerned about how an employer might use the work you’re doing, you can ask, ‘Can you tell me how you’ll use the work I produce? Is it for evaluation purposes only?’”