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 (subsite.mainsite.com) 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 (www.mainsite.com) 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?