Poner a 0 todos los artículos de un distribuidor:

UPDATE ps_stock_available AS s 
INNER JOIN ps_product AS p ON p.id_product = s.id_product 
SET s.quantity = 0 
WHERE p.id_supplier = 7; 

Modificar también el precio en este caso según una referencia:

UPDATE ps_stock_available AS s
INNER JOIN ps_product_shop AS ps ON ps.id_product = s.id_product
INNER JOIN ps_product AS p ON p.id_product = ps.id_product
SET s.quantity = 0, ps.price=0 WHERE p.reference = 'A90210';