How to restrict plugin activation in sub-blogs in a multisite environment

Hi All

I have a site based on MU, with buddypress and multisite. I have some plugins in the main/parent off all that are very spcial and developed for the main site.

Now, The subsites ( because they are MU, can have their own users and they can create their own blog.

When you create your own blog in a subsite you get to be the administrator of that blog but now you can see the plugins we have in the main site ( and you have the option to activate them. I can not have that. Even more, I need to prevent any blogger to upload a plugin.

How can I ever do this? Ideas?