Disable registration on all blogs except main one

Hello!

I know that when a user clicks the register link on any blog he is redirected to the main website to register. I would like to disallow this and hide all the register links. So that for registering in the network the user would have to to go to the main website.

Still, I would like to allow the normal login for the administrators and such.

Is there an easy way to do this?

Thanks!