Block Users from Sub-Site1 from Login in Sub-Site2

Hi there heroes.

Here's something we were not expecting: When a user register on Sub-Site 1 of our WPMU, he will also be able to login in any sub-site (sub-site2, 3, 4 and so on).

Is there any way we can block this? If a user register on sub-site 1, he want him to need to register again on sub-site 2 to login.

Thanks for your help on this!