A blocker apparatus (20) for blocking a stream (22) of high-pressure fluid includes a blocking member in the form of a sphere or ball (30), or a rod (160) receivable within a partially surrounding seat (34/162) at the forward end of a seat carrier (40) to rotate in the seat. The seat carrier (40) is extended and retracted by an actuator (32) to place the blocking member within the path of the stream (22) and/or remove the blocking member from the path of the high-pressure stream. The blocking member is replaced by simply lifting the blocking member (30/160) off of the seat (34/162) and lowering a new blocking member into the seat.