block mapping on main site domain

Hi,

I am running a WP subdomain network with an adress like http://www.mynetwork.com

And I noticed that it is possible for a subsite admin to map mynetwork.com (without www) to their subsite, which is not good for many reasons.

What could I do to prevent subsite admins to set mynetwork.com as mapped domain for their subsite ?

Thanks for your help.