Owners of public Mastodon servers, you might want to set your signups to approval mode to prevent spammers:
1. Log in on your server's website
2. Go to Preferences
3. Go to Administration
4. Go to Server Settings
5. Click the Registrations tab at the top
6. In the "Who can sign up" menu select "Approval required for sign-up" (optionally also tick the box for requiring a reason)
7. Click "Save Changes" button
Spammers can exploit servers with instant signups.