Proverbs 17:10 A reproof entereth more into a wise man Than an hundred stripes into a fool.
Which are you; a wise man or a fool? How do you deal with reproofs?
In discipleship you will soon know who you should work with. If he handles reproof well then maybe he can be trained. Others are so hard headed that you will accomplish very little with them.
I think of it like putting the ball in their court. Give them a chance and see what they do. If you talk to them and show them what is right and wrong and then all they want to do is argue then you might have to just be nice to them and look for someone else to train. If he handles the reproof you can continue on with him.
Give an assignment. Wait to see if it is done. If after several attempts you see no results leave him and move to the next man. Do not be ugly just move on and wait for God to work in his life. When he is ready he will respond correctly.