"Goblin of Greed" prevents discarding as a cost, but not as part of an effect. So "Goblin of Greed" will stop "Kuriboh", but not "Mirage of Nightmare" or "Graceful Charity".
If your opponent activates an effect and discards as a cost, and you chain "Call of the Haunted" to Special Summon "Goblin of Greed", he doesn't stop the effect because the cost was already paid.