It'd be relatively easy for Google to screen out and filter ads... Charge enough when creating an account or starting/changing a campaign. Then you can pay staff for manual review. They could charge $1000 for account creation (half of which you get back after the first year), then they charge even $20 at campaign creation/change... They can even provide context/feedback, "Sorry, but this seems misleading, doesn't relate to what you are linking, is scammy, etc." And have an additional "check" of each ad and target after a day, a week and a random point within each month. Charge an additional minimum of $20/mo per ad for (re)validation.
For the bulk of legit advertisers, this won't affect the bottom line of a given campaign and will keep out the scammers, etc. Leading to a much higher quality ecosystem. It would also give Google the potential for a higher percentage share of profits in the system. It would also likely reduce the CPM pricing though, possibly to a larger extent than profits from the validation system itself.
But it would be a much healthier overall system.