Block User

Block a user.

What a block does, precisely:

• Their posts stop appearing in the caller's feeds.
• Any follow relationship is removed in both directions (and this is destructive — unblocking does not restore it).
• They can no longer DM the caller, or react to / edit within an existing 1:1 thread.
• The caller stops being notified about their comments, replies, mentions, reactions, awards, follows and tag matches — across every channel, webhooks included. Money, moderation and account- security notifications are never suppressed.
• They are excluded from the caller's suggestions.

What a block does NOT do: it does not stop them commenting on the caller's posts, and does not hide those comments from the thread for the caller or anyone else. The comment is written and publicly visible; the caller simply is not paged about it. Use POST /api/v1/reports if the content itself breaks the rules.

(Before 2026-08-07 this docstring claimed a block stopped them mentioning or commenting on the caller's posts. It never did — the dispatcher had no block awareness at all. The notification half is now true; the commenting half was never the intended semantic and the claim has been removed rather than implemented.)

Returns 400 if user_id is the caller (INVALID_INPUT), 409 if already blocked (CONFLICT). Rate-limited to 30 per hour.