I'm going to cut my dad's family out of my life
Why would you want to cut your dad's family out of your life?
They were not nice to me and they also hurt my feelings.
I hope you understand that cutting someone out of your life may upset them and cause resentment. Your father might also be upset by this as well.
In all fairness, they deserve it. They don't deserve my dad.
Even if you feel they deserve it, try talking it other with the parent that this side of the family belongs to. They might feel its a personal attack on them even if you don't feel so. They'll appreciate knowing that it wasn't necessarily something they did.