Cómo crear un activador para eliminar php myadmin

Debe establecer el delimitador en algo diferente a ;

DELIMITER |
CREATE TRIGGER blockuserundo BEFORE DELETE ON user_blocked
FOR EACH ROW BEGIN
UPDATE members SET ty_status = '1' WHERE user_id = OLD.block_user_id ;
END;
|
DELIMITER ;

El siguiente código funciona como se esperaba:

DROP TABLE IF EXISTS user_blocked;
CREATE TABLE user_blocked 
(
    block_user_id INT
);
DROP TABLE IF EXISTS members;
CREATE TABLE members 
(
    user_id INT,
    ty_status CHAR(1)
);

INSERT user_blocked
VALUES (1),(2),(3),(4),(5);

INSERT members
VALUES (1, '0'),(2, '0'),(3, '0'),(8, '0'),(9, '1');

DELIMITER |
CREATE TRIGGER blockuserundo BEFORE DELETE ON user_blocked
FOR EACH ROW BEGIN
UPDATE members SET ty_status = '1' WHERE user_id = OLD.block_user_id ;
END;
|
DELIMITER ;

SELECT * FROM members;

DELETE FROM user_blocked WHERE block_user_id IN(1,2);
SELECT * FROM members;