How to remove or change a validation rule after it's been se

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