Bullying is wrong because it is sinful behavior.  We are supposed to 'Love Thy Neighbor.'  We also 'Love Thy Neighbor' by suggesting to him ways to avoid Hell, and that is to not commit Mortal Sins.  So, there is a nice way to teach somebody.  There is the book of Tobias which teaches us the purpose of a man and woman and that is procreation.  That's been Catholic teaching for almost 2,000 years.