BuddyPress.org

What, if any, plugins are you using? what versions of WP & BP are you running.

When you say user of ‘Main’ site what exactly do you mean, all members are subscribers to the main blog that runs BP. When you say that they can ‘see your admin’ how are you establishing that fact; if for instance you, logged in as ‘Super Admin’, go and visit the backend/dashboard of a user blog from the ‘Sites’ list you will see the Super Admin menu, but not if you log in as the real user. If behavior is different than this it sounds like you have plugins or customisations at work?