ActiveRecord: removing validations or callbacks on a subclass

Often a parent class has validations (or callbacks or associations)
which I don't want the subclass to have. Is there any simple way to
remove these?

Not really. Just don't add them. If you have 5 sub classes, and only
3 of them need the validations or callbacks, set it up in a special
mixin:

module PickyCallbacks
  def self.included(base)
    base.after_save :foo
    base.validates_format_of :email, :with => /.../
  end
end