knock->knock and rescinding of knocks are not allowed

proposals/2403-knock.md

@@ -36,8 +36,8 @@ There are restrictions to being able to set this membership.
 
 ### Current membership
 Only users without a current membership or with their current membership
-being "leave" can knock on a room. This means that a user that is banned or
-currently in the room cannot knock on it.
+being "leave" can knock on a room. This means that a user that is banned, has
+already knocked or is currently in the room cannot knock on it.
 
 ### Join Rules
 The `join_rule` of `m.room.join_rules` must be set to "invite" for a knock to
@@ -74,6 +74,14 @@ that user can be transitioned to the following possible states:
  - `ban`: In this case, the knock was rejected and the user has been prevented
    from sending further knocks.
 
+Users are not allowed to change their membership once set to `knock`, in
+order to prevent users from being able to knock multiple times and spam a
+room.
+
+XXX: So if you knock on a room that's then abandoned that's in your `/sync`
+forever? Clients should have a way to tell their server to hide and show
+knocks.
+
 ## Client-Server API
 Two new endpoints are introduced in the Client-Server API (similarly to
 join): `POST /_matrix/client/r0/rooms/{roomId}/knock` and