You can either find the built in elf ears morph and set it to negative, or find whatever morph the model uses and set it to negative - if the model uses one custom body morph you need the morph merger plugin which get split the morphs back and you can check which elf ear morph is being used. It is a safe bet that most use Damarmau's morph pack, so you can set the "Damar Elf Ears" morph to negative.