@deiden26 it's not a simple change to accomplish that. Only the categories ignored are maintained at the user level.
You can create a plugin to listen for action hooks on new category creation and new user creation to do that.
action:category.create... get a list of all users and call
User.ignoreCategory for each user to add it to their ignoring categories list.
action:user.create... get a list of all categories and call
User.ignoreCategory for each category.
Obviously for all existing categories and users, you will have to write a one time script to do it.