Link: https://www.jefftk.com/p/thinking-about-mastodon
To be honest, I don't think this will ever become that much of a problem. Because most instances admins care about moderation (and obviously don't allow spam inside their walls), only a few spammy instances should be an issue, and they can easily be blocked.
People can of course use another instances (or host their own) to keep sending spam, but this adds an expressive burden. Not much different from Twitter, where people can (and do) create multiple accounts.