If you want to block them yourself, you can do so once they go live by selecting "Block <domain>" in the context menu of one of their posts or accounts.
If you want to block them ahead of time, you can do so via the CSV import. On https://chaos.social/settings/import, select "Domain blocking list" and upload a file containing "threads.net", (and to be safe, "threads.instagram.net").
Here's a file like that: https://gist.githubusercontent.com/rixx/cff5003c5054b9288d67552f544d26bc/raw/e81dd45648367b0ace0646675255f2b154368dd0/blocked_domains.csv
You can see your blocked domains at https://chaos.social/domain_blocks