locking themes in multisite

Hi, I am trying to setup a multisite where after I create the first example site, I was going to use your New Blog templates plugin to replicate example site after users sign up for a new blog (was going to use Blog and User creator for this).

So on my network I have a few themes installed, but I only want to allow microsites access to the one theme I setup in the new blog template. In other words I dont want them to have access to the theme I am using for the main "parent" blog site.

What is the best way to do this? Is there a way to either manage themes on a blog by blog basis? or even make the appearance panel disappear, though this poses the added issue of users not being able to edit the widgets.

Thanks!