WordPress MU Howto control theme to customer assignment

Hi,

I have installed latest version of WP MU. Now I have multiple customers running on this server. Is there any way how I can define that only one customer can see a specific theme. The problem is that when I have a custom theme “one” for customer “one”, then customer “two” should not be able to use the theme of customer “one”. Is there any plugin for that? Actually I can only activate a theme for the entire network.

Cheers!