Block the other writer and end the exchange

As leave, and the two accounts are never paired again. The block is recorded against the other writer's alias, so the caller's block list never shows who they were, and a hash of the other writer's verified phone number is kept so a new account with the same number is not paired with the caller either. Works after the exchange is over too. In a group exchange (from the recruitment board) member -- the other writer's alias display name -- says who, and only the caller steps out; an exchange of two needs no body.