first of all stop all internet connections and then delete the unwanted contacts . Then save the list as a vcf file. After a fresh install of the Rom just go to settings>Acc & sync settings > disable Auto Sync and never try to sync contacts also uncheck automatic restore in backup settings.( if you have a back up of the correct contacts list, then go to apps> contacts> clear data and also clear the data of contact storage then restore the vcf file after disabling Auto Sync)
so, never sync ?