Rules:

• Allow blocking a profile using either Profile ID (UUID) or Profile Custom ID.
• Exactly one of these must be provided. Supplying both is invalid.
• The provided Profile ID or Custom ID must belong to the same application as the blocker (based on the auth token). If not, return HTTP 400 with the message "Blocked profile not found".
• If a block relationship already exists between the two users, return HTTP 409.
• Optionally, if the client wants to remove the relationships between these two users, along with blocking, in both directions, they can include the remove_relationships field in the request payload.