That is easy. The ad placers are liable for 100% of the harm caused. They can figure out how to deal with it so they stop causing harm. A simple solution is re

That is easy. The ad placers are liable for 100% of the harm caused. They can figure out how to deal with it so they stop causing harm. A simple solution is requiring the companies providing the ads to be placed to indemnify the ad placers. That means the ad placers push the monetary liability onto their customers to only provide legitimate ads. As they know exactly who their customers are and can vet them it is trivial for them to go after them if they are sketchy. Easy. Next problem.