Last I checked, this is still only available on PC, not through the mobile app.
Step 1: Click the “…”
Step 2: “Block”
**That blocks an IP address (the unique internet address of a house). The anon could still reach out via mobile device or through like a school/library computer, so you may have to block them a handful of times depending on computers they have access to.
Blocking anons doesn’t register in your “Blocked” list, so it cannot be undone. That’s more of a warning to people sending the anon. If you’re just joking in a snarky voice, remember that tone doesn’t come across on just flat text and you need to be conscientious of how your words might read to others.
Also, your inbox may still show a counter for the blocked message. Get some friends to help spam your page with 3-4 messages and below the inbox menu you’ll see “Delete All Messages”. Clicking that also wipes out the blocked anon message.