First, respond to every inquiry by telling them the price is USD$70,000,000.00. And stick to that price. Many of these sleazy companies get their leads from the same "lead generators," who will eventually take you off their lists because they know your terms are unreasonable. It doesn't work for everyone, but when I did it to spammers trying to buy my mailing list, it significantly reduced the volume of inquiries.
Second, put a page on your web site listing all of the offending companies, with links to the letter you received.
Apr 1, 2021 - Company X promised $3-5k/month if I alter your search results. Link.
Apr 3, 2021 - Company Y promised $1-5k/month if I promote thier product on other people's web pages. Link.
A lot of people on HN will claim "O, noes! Lawyers! Libel!" I wouldn't worry about it. These people don't have the money for lawyers, are usually in geographies without legal systems, and don't want their names and other information exposed in a public legal filing. Plus, all you're doing is stating facts.