hm, I don't like very much that option, keeps us away of having simpler code. I'd prefer to stick to a warning message in the docstring for this module.
A possible way to avoid the creation of new flags can be found in here:
http://stackoverflow.com/questions/3603502/prevent-creating-new-attributes-outside-init

We would have a class instead of a module, which is not as simple as this.
In my opinion we don't need to be THAT careful.