Block a remote user or instance from a group

[mayel]

"As a group admin, I want to block a remote user or even an entire remote instance from participating in my group, so I can protect members from coordinated harassment."

We need group-level blocks separate from instance-level blocks.

Remote moderators can manage blocks following the same pattern as other moderation actions:

1. Mod sends Block to group inbox
2. Group verifies mod has permission
3. Group updates internal block list

{
  "type": "Block",
  "actor": "https://remote.example/users/mod",
  "object": "https://bad-actor.example/users/spammer",
  "to": "https://example.org/groups/1"
}

To block an entire instance, the object could be the instance's actor or domain:

{
  "type": "Block",
  "actor": "https://remote.example/users/mod",
  "object": "https://bad-instance.example/",
  "to": "https://example.org/groups/1"
}

Implementation behavior:

• Posts from blocked actors/instances are not Announced
• Follow requests are auto-rejected
• Existing members from blocked instances could be removed or grandfathered
• The block list acts as an exclusion that overrides permissions in interactionPolicy

Privacy of block lists

Block lists should NOT need to be publicly exposed:

• Revealing blocks can enable harassment evasion
• The group's home instance enforces blocks without publishing them

Remote instances discover they're blocked by attempting actions:

• Follow → Reject (no reason required)
• Post submission → silence (no Announce, no explicit rejection required)