If you're an admin using Mastodon 4.0.2 or below, the menu described before won't exist and you will need to access your Mastodon server's #PostgreSQL #production #database (assuming you're using PostgreSQL) and paste the following #SQL code to block threads.net:

INSERT INTO domain_blocks (domain, created_at, updated_at, severity, reject_media, reject_reports) VALUES ('threads.net', now(), now(), 1, 't', 't');