If the Fediverse becomes sufficiently popular, the following might happen:

1. Advertisers start using tags or direct messages to get into people's eyeballs.
2. We can block individual accounts, or servers that allow this sort of thing, but it's easy to set up new servers.
3. Moderators (or maintainers of public block lists) adopt a defensive policy of not federating with any server that hasn't been around and active (with believable human conversations) for at least, say, a year.
4. This makes it very hard, but not impossible, for individual people to set up new servers; they just have to create a community through some other means and hang out in their isolated space (or with each other) for the first year.
5. An industry arises of using AI to create thousands of fake "communities" that, once federated, let loose with the advertising.

Alternately, after step 1, we might start using anti-spam software to filter our tags/DMs the same way we currently filter our email.