The Cetra actually tried to fight off Jenova -- not J-E-N-O-V-A, mind you, because that's the instrumental music piece -- but they had no luck and the "virus" spread across the Planet, turning the Ancients into the monsters in FFVII. The few surviving managed to seal Jenova, and the Lifestream, not the Cetra, tried to heal the so-called wound that Jenova's crash-landing caused.