Comment by NeutralCrane

I don’t think it would be onerous nor ill defined. Simply make it illegal to pay someone to or receive payment for making a public announcement for a product, service, or brand. If no payment is involved, it’s fine. People are free to promote their own or others products on social media, YouTube, the side of their car or house, so long as they aren’t paid to do so. That is hardly any more convoluted or ill-defined than dozens of other laws on the books.

And yes, word of mouth and non-paid advertising is absolutely capable of spreading awareness on its own.