How do you moms discipline a toddler that doesn't seem to understand his boundaries? My son doesn't…

How do you moms discipline a toddler that doesn't seem to understand his boundaries? My son doesn't listen if I say firmly "NO" doesn't understand if I spank him (which I rarely do) doesn't seem to understand if I take things away. I'm honestly reaching my breaking point. Today he threw at me one of his plastic toys and it landed on my nose and bruised the bridge of my nose. I've been having a slight headache ever since. I did spank him for this, and he tried pulling my hair. I don't get it.