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"Cursed Seal of the Forbidden Spell" only negates the activation of Spell Cards. It does not prevent the use of Continuous Spell Cards already active, like "Archfiend's Oath".
If you activate a Spell Card and chain "Cursed Seal of the Forbidden Spell", then your opponent cannot activate Spell Cards of the same name, even though your opponent didn't activate the Spell Card in the first place.
When you negate and destroy "Black Pendant" with "Cursed Seal of the Forbidden Spell", and afterwards a Set "Black Pendant" is destroyed, the effect of "Black Pendant" will not be activated (neither the "Black Pendant" that was negated nor the Set "Black Pendant" destroyed later will inflict 500 damage).
The effect of "Cursed Seal of the Forbidden Spell" lasts for the entire duration of the duel.
If "Fiber Jar's" Flip effect has successfully resolved, Spell Cards affected by "Cursed Seal of the Forbidden Spell" are still not allowed to be activated for the remainder of the duel.