This isn't, and has never been a hard problem. Just pay for people's attention. People you follow don't have to pay, and make that transitive. Penalize people in your network who propagate spam by increasing the cost to get your attention.
This is one of the most interesting properties of peer-to-peer networks.
You can run your own ingestion algorithms, and one of the things you can do is set up inbound rules that incorporate micro transactions.
We have to build a lot of infrastructure to make this work, but it seems ideal for a world full of agents and autonomous systems acting on our behalf.
If a scammer, advertiser, or some other form of spammer can get a payout just 1% of the time, they will be willing to pay much more than the average person posting the average tweet.
If you make everything explicitly transactional, you will be left with only people trying to make a profit.