How to remove the register function from the network and offer it only on main Blog?

Hello!

I have the problems that - mainly spammer - people register themselves on the Blogs of my WPMU Network, but not on the mainblog, by the knowledge of the deep link to the register on every network site!

How can I remove the possibility to register - for external people - on the network sites, so that the people can only register themselves on my main site?

Thanks for your answers and help!