In the years I have been doing take-home exercises for candidates, I have yet to see a single instance of a great take home leading to a terrible on-site or hire in terms of technical skill. There have definitely been great take-homes for candidates that we later chose not to hire, but usually due to challenges interacting with the product team, or in some cases too much fixation on technical perfection with no focus on business realities, etc. If there are a large number of candidates paying others to do their take homes for them, they should be asking for refunds.
Second, even if this is a problem, there's a very simple solution. For candidates that do well on the take home, either on a call or on the on-site, do a live code review with the candidate and ask them to explain a bit more about why they wrote the code the way that they did. Ask them questions about other approaches, or what they'd do if they have more time. Candidates will be significantly more comfortable talking about their own code they wrote. It'll be quite clear if the candidate didn't write it themselves.