FFS is the acronym "FOR FUCK
SAKE" and you can
basically use it anytime you get disappointed, be creative.
BUT, if you'd like to be a bit more international than that you could say "joder" in spanish, AND if you'd like to be more creative in spanish, you could say "por el amor
de Dios".
"FOR FUCK SAKE (FFS). You're
writing in my column!" or "¡Por el amor
de Dios! Estás escribiendo en
mi columna."