Blocks one account: its risk state becomes BLOCKED and all of its sensitive events return denied. Allowed from NORMAL, QUARANTINED, and RECOVERING. You do not need a previous denied: block whenever your own reasons say so. The only way out of BLOCKED is a recovery.

Safe to retry: if the account is already in that state, it returns 200 with the existing resource (the same id), without side effects or a new history entry. A retry with different data (for example, another reason) does not change the account or the recorded reason.