안 돼요 itself simply means "may not" so the subject really can be anything. It depends on the context. (저는) 이 책을 사고 싶어요 안 돼요. (I) want to buy this book. You can't. (You may not.) 저희 아이들이 먼저 들어가도 돼요? 안 돼요. Can my kids enter first? (They) may not.