WPMU – New registered users are auto registered on susbsites

Hi,

I’m having the following problem.
I have a WP Network and every new user that registers on my main site auto-registers on each network subsite that he visits. Eg. The user1 creates an account on the http://www.mysite.com/wp-login.php?action=register and once he creates his accounts he only has the registered user for mysite.com, but as the website displays posts from some subsites he also gets registered on those subsites, then if he visits a different subsite, say mysite.com/subsite9 he also gets registered as a user for that subsite. I don’t want this to happen, anyone knows how to fix this?