Not sure if I understand your use case correctly but I'll give it a go:

If by reverse the inheritance you mean having parent pages inherit from their children? How would that work if the property is set on multiple children? I would look to other options like a configuration setting instead.

Anyway if you want a dynamic property to exist on all pages but one you can add a property with the same name on the child page and setting that value will "override" the value of the dynamic property without loosing the inheritance to other child pages.

If you want to cancel the inheritance you would need to set the dynamic property on the child page. If you really need it empty I would recommend using a specific value that represents an empty value, like '-' or similar depending on the type.

Cheers
Henrik

Mats, a page will always inherit it's parents dynamic property value. I've written a blog post about it here:
http://www.epinova.no/no/Blog/episerver_blog (Custom boolean property) 

And the problem is also beeing discussed here:
http://world.episerver.com/Forum/Pages/Thread.aspx?id=13118&epslanguage=en