If someone has talent then you should stick with them because they can surprise you and you can actually learn a lot.
My first ever student was the most intolerably selfish, moody and contrary girl you could ever wish not to meet. She clearly had ability but was severely maladjusted. I stuck with her for about six months before I got to the point where I was ready to throttle her. I then implied to the mother at the end of one particularly awful day that her time was running out, so to speak. I was ready to jump ship at the soonest opportunity.
Well, I'm not sure what happened that evening but clearly mother and daughter had words because the following week she was far more co-operative and stayed that way for weeks afterwards.
Within a month or so I finally started to get through to her and the situation turned around completely. She began to look forward to lessons, practiced of her own free will and eventually became someone I really cared about. I ended up teaching her for five years, almost without break.
The important thing is that not only was that a life changing experience for her, it was for me too. After going through that, teaching 'normal' kids seemed like plain sailing. It was a trial by fire and what I learnt from that experience made me a vastly better teacher. I've rarely had problems with children ever since.
You don't learn anything at all from teaching co-operative students. Anyone can teach a polite, eager to please child - no special personal skills are required. But if you consider yourself to be a good teacher, you should be able to teach anyone. And even if it seems unpleasant or impossible, you owe it to the child to give it your absolute best shot if you see potential there.
In my experience, a reasoned but straight-to-the-point discussion with parent and child is the starting point for dealing with difficult students. Both child and parent feel embarassed, which becomes a motivating force in improving behaviour. You've then got to get through to the student, and find out what motivates them and what they like.
For the ultimate inspiration in dealing with children with behavioural problems, read Ann Sullivan's moving blow-by-blow account of how she dealt with the infant Helen Keller:
https://www.amazon.com/Story-My-Life-Unabridged-Centennial/dp/0393057445/ref=pd_bbs_1/103-7672824-7340646?ie=UTF8&s=books&qid=1189423764&sr=1-1