止痒 is a separable verb-object (VO) compound meaning 'to stop or relieve itching'. The verb 止 ('stop') can be separated from the object 痒 ('itching') by inserting aspect markers or complements, e.g. 止住了痒 ('stopped the itch'). You cannot put a direct object between them because 痒 already fills that role; instead, you modify the VO unit with adverbs or complements around it.