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