Please do not pick the kitten up by the back of the neck! If done improperly you can cause internal bleeding. The only ones to pick up a kitten that way should be it's mother or people who know how to properly support the kitten's back legs at the same time as you scruff. I can't believe a vet told you to do that.
Before anyone can give you advise we need more detail. What is the kitten doing when acting aggressive? What are you doing more importantly? How old was the kitten when taken from mom? Any siblings? Etc, etc......... The more detail the better.
Assumptions do nothing but make an ass out of u and me.
We can stick our heads in the sand for only so long before it starts choking us. Face it folks. The pet population is bad ALL OVER THE WORLD!