How to remove validation using instance_eval clause in Rails?

I would like to enhance existing class using instance_eval. There original definition contains validation, which require presence of certain fields, ie:

class Dummy < ActiveRecord::Base
  validates :field, :presence => true 
end

Now I want to change that to optional using instance_eval (or any other method, really):

Dummy.instance_eval do
  ...
end

What would be the proper syntax to remove the validation, so the field is optional. I would rather do this directly on the model layer, instead doing weird hacks in controllers or views. The use of instance_eval is not really required, but as far as I know, this is generally the best way to enhance classes in Rails.

Edit #1

In general - the original class is part of the gem and I don't want to fork it, nor tie to specific release. The general cause is not really important. Simply editing the original model has far worse consequences than monkey patching.

I found a solution, not sure how solid it is, but it works well in my case. @aVenger was actually close with his answer. It's just that the _validators accessor contains only information used for reflection, but not the actual validator callbacks! They are contained in the _validate_callbacks accessor, not to be confused with _validations_callbacks.

Dummy.class_eval do
  _validators.reject!{ |key, _| key == :field }

  _validate_callbacks.reject! do |callback|
    callback.raw_filter.attributes == [:field]
  end
end

This will remove all validators for :field. If you want to be more precise, you can reject the specific validator for _validators which is the same as the raw_filter accessor of validate callbacks.