How does a visitor register with an individual blog, but NOT with the site admin?

I have a site installed where my bloggers would like to register their visitors as subscribers to their individual blog. They do not want their visitors to see the "opportunity" to get their own blog. They figure it is my job to attract other bloggers. They want their visitors to be their subscribers, only.

When the blooger goes to and clickes on "Register," they are taken to and given a choice of subscribing or getting their own blog.

That is not what we want.

What can I do to ensure that the visitor only gets to signup for the subdoamin (blog) ?

Thanks in advance,