User Registration Question - I'm getting too many spam users!

So I have a WPMU install
some of my sub-site I want to allow user registration
however I am getting too much spam!
and I want to prevent registration on my main MU site install! So I have disabled new user registrations but now my hosted sites are unable to allow new users to register as well! I thought the Membership plugin would fix this but it does not! I need a good way to manage my memberships both on the MU site and on the individual sites. Is there a way to have it enabled on one and disabled on the other? Otherwise I'm going to have to scrap the MU idea and just move these sites (back) to individual WP installs. Please advise the best method for dealing with both MultiSite registration and individual site registration and also for preventing spam member registrations (I get about 5 per day)