It's possible that the business could be so successful that everyone uses it, the services selling this information all run out of customers and go out of business, none of them come up with newer and more evil ways to do this, and you run out of potential customers. In which case:
mission accomplished, retire on your giant pile of money and bask in the knowledge that you made a far better place. (Avoid scenarios in which you have perverse incentives to allow the problem to continue.)
But in the meantime, tens of millions of potential customers times any reasonable fee seems more than enough to build a substantial business on.
You could tempt people in with a cheap fee to let them send in a few pictures of junk mail and stop those. (As you get more, find the biggest sources and automate or batch them so that they cost you almost nothing, which will pay for the higher-effort ones. Have an upper bound on effort expended, and tell people that they don't pay if you can't remove them.) You could then track down the underlying sources, and if you successfully identify them, contact the customer, and give them enough information to decide to pay you for a higher-end service to get them removed from those sources (and keep them removed).
The value that gets people to keep paying you would be a steady stream of reports of "we found this source leaking/selling your information, here's what we did about it". It'll take you years to track down all such sources and find paths to remove them; you will likely end up having to fund some legal work and possibly even a lawsuit or two, which will give you a giant pile of publicity.
(As one example of something much easier for a company optimized for the process to do than an individual: the USPS has a detailed process for formally putting a company on notice for mailing someone who has specifically unsubscribed, and that process ends in massive fines for continued mailing to that person. I read a report of someone doing that to stop receiving persistent Dell catalogs.)
If you're sufficiently creative, you could even pitch this as a service to marketing companies. You have a list of people who will not buy anything via direct mail, and who will despise any company that they receive such mail from. Convince the sources of postal spam that removing those people from their list makes the rest of their list more valuable. Convince the downstream customers of those sources that using your list directly is far more convenient for them than dealing with opt-outs from every individual on it.
That also gives people a continued incentive to pay to remain on that list.