WorldgenRemoveEventJS (1.16)

For more information on biomes field, see worldgen.add event page.

onEvent('worldgen.remove', event => {
  event.removeOres(ores => {
    ores.blocks = [ 'minecraft:coal_ore', 'minecraft:iron_ore' ] // Removes coal and iron ore
    ores.biomes.values = [ 'minecraft:plains' ] // Removes it only from plains biomes
  })
  
  event.removeSpawnsByID(spawns => {
    spawns.entities.values = [
      'minecraft:cow',
      'minecraft:chicken',
      'minecraft:pig',
      'minecraft:zombie'
    ]
  })
  
  event.removeSpawnsByCategory(spawns => {
    spawns.biomes.values = [
      'minecraft:plains'
    ]
    spawns.categories.values = [
      'monster'
    ]
  })
})

If something isn't removing, you may try to remove it "manually" by first printing all features (this will spam your console a lot, I suggest reading logs/kubejs/startup.txt) and then removing them by ID where possible.

onEvent('worldgen.remove', event => {
  // May be one of the decoration types/levels described in worldgen.add docs
  // But ores are *most likely* to be generated in this one
  event.printFeatures('underground_ores')
})

onEvent('worldgen.remove', event => {
  event.removeFeatureById('underground_ores', 'mekanism:ore_copper')
})