Rails - Feb 2006 - how to remove or change a validation rule after it''s been set?

I''m using the login_engine in a project, and I want to customise some
of the
validation rules that the User model applies.  It seems that I can add more 
validation rules by re-opening the User model class in a new model file under 
app/models, but how can I remove or change rules that are applied in the 
engine''s code?

Thanks

Luca