Extinction

Extincting a type of monster, as the name suggests, means killing enough of them that no more will be generated. For most species, the magic number of created monsters is 120; for erinyes it is three and for Nazgul it is nine (as would be expected). Once a species has been extincted there are very few ways to create new monsters of that species - there are exceptions, such as pudding division, summon nasties, etc.