Adding a Theme to your site that can only be used on the Main network page

Hi All.

Is there a way to add a theme that you only want the main site to be allowed use ?

The reason is i've paid for a great theme.. and the license is only for 1 site ( my main site ) and i don't want everyone that signs up to the free blog ( multisite ) be able to use it.

Is this possible ?

thanks.