How can I disable a validation and callbacks in a rails STI derived model?

Question

Given a model

class BaseModel < ActiveRecord::Base
  validates_presence_of :parent_id
  before_save :frobnicate_widgets
end

and a derived model (the underlying database table has a type field - this is simple rails STI)

class DerivedModel < BaseModel
end

DerivedModel will in good OO fashion inherit all the behaviour from BaseModel, including the validates_presence_of :parent_id. I would like to turn the validation off for DerivedModel, and prevent the callback methods from firing, preferably without modifying or otherwise breaking BaseModel

What's the easiest and most robust way to do this?