How is Mastodon able to defend against abusive single-message throwaway accounts registered by a distributed swarm of malicious human beings? My stalker of 20

How is Mastodon able to defend against abusive single-message throwaway accounts registered by a distributed swarm of malicious human beings? My stalker of 20 years has been registering a new account every time they contact me for 20 years, specifically to abuse the “your first message is trusted and delivered” approach, with every social platform they’ve stalked me on. If my stalker has known how to do this for decades, then clearly these forum folks know how to as well. By my read, Mastodon is vulnerable to this distressing and threatening behavior as long as each account is treated as a throwaway, even if it’s banned as soon as it’s caught. This allows the flash mob of abusers to sign up for a mass of Mastodon accounts, send a single threatening abusive message from each to one recipient, and then throw away the accounts and start over. They would succeed in their targeted harassment goal, while Mastodon would - as it defaults to “allow untrusted third parties to contact anyone” - continue acting as a delivery platform for abuse that cannot be stopped. If I’ve misunderstood and there is some aspect of Mastodon that protects against anonymous users being treated as innocent long enough to deliver an abusive message, that would be invaluable to know.