I have access to the backend of my site restricted to Administrators and Editors but I also

I have access to the backend of my site restricted to Administrators and Editors (of the main site) but I have also created additional user-roles with user-role-editor but that shouldn't compromise access to the Admin section of my site.

One of my members created a sub-site and as such he's an Admin on his site (but) now he has full access to the back of the (Main) site... Scary thought when we have thousands of subsites...

What do I do per se???!!!