The problem with adult scale is that they are covered in a hard shell that protects them from spraying. Many people use a toothbrush dipped in insecticidal soap or other insecticide and gently go over the plant and dislodge them. Or just squash them with your finger and then spray. Immature scale can be killed easily with insecticide, but they are highly mobile. Be sure to spray the entire plant especially undersides of leaves, the crown area, and even the top of the potting medium. Spray several times, several days apart to kill newly hatched insects.