Dyer seemed set for a switch to Upton Park after the Hammers had agreed a £6million fee, but someone put a spoke in the works.
And now Curbishley insisted the deal is now "definitely off".
Curbishley: "We made an offer, had it accepted, agreed personal terms and the player passed a medical but it was pulled at the 11th hour by Newcastle and there's not much we can do about that.
"It has never happened to me before. We are disappointed and the player is very disappointed but it's definitely off."
It is thought Newcastle had asked West Ham for more money after the London club had been promised £2million in the settlement over Carlos Tevez.