Can Cursed Seal of the Forbidden Spell negate effect a spell card already on field in Yu-Gi-Oh?

Yes, Cursed Seal of the Forbidden Spell can negate the effects of a Spell card already on the field in Yu-Gi-Oh. Cursed Seal of the Forbidden Spell continuously negates the effects of all set Spell cards on the field. So, if a Spell card is set and activated on the field, Cursed Seal of the Forbidden Spell will negate its effects when it resolves.