Cursed Seal of the Forbidden Spell doesn't work on The Seal of Orichalcos, does it?
No? Why does Seal of Orichalcos being a field spell, mean that people can't negate the activation with Cursed Seal of the Forbidden Spell? Or are you talking about its protective effect? Still won't work. Just so we're being clear, you 'activate' Seal of Orichalcos when you place it face-up in the Field Spell Card zone, or when you flip one that was Set there. You can chain any spell card activation negation cards to this (a field spell is still a 'spell card'), such as Cursed Seal of the Forbidden Spell. It will negate the activation, destroy the card, and in this case, prevent any more Seal of Orichalcos being played that duel by that player. (Seal of Orichalcos already has a condition that says only one can be activated per Duel. Normally, if the activation is negated, then you are allowed to activate a second. It just so happens that in this example, Cursed Seal would shut down that possibilty also because of its effect.) Secondly, Seal of Orichalcos has an effect that says "Once per turn, this card cannot be destroyed by card effects.", this effect can only be applied after it has resolved to the field. It cannot stop destruction from an activation negation card. If you've already resolved Seal of Orichalcos to the field, then it is indeed safe from Cursed Seal. for two reasons. One, it has no further activated effects that things could be chained to (for example think of Royal Oppression, which resolves to the field and has a repeatable Quick Effect) all of its subsequent effects work continuously without using the chain. Secondly, even if it DID have further activated effects, Cursed Seal of the Forbidden Spell is not a card that can stop those, it can only stop the initial activation. Lastly it also has its protective effect to ignore the first destruction attempt.