I think if you type baller and make sure to click the word so it doesn't auto correct, and you type it three or four times in a row, the phone will stop changing it to ballet.
Type "balla" instead of "baller". It's more "baller" to put "balla" anyway...
As one mentioned.. just correct the auto-correct. Hit the x when it goes to correct and if done routinely it will begin to learn baller instead of ballet.