WordPress MU

I installed this plugin to my wpmu website..

It adds the new option under the admin > option and I placed a value of 1.

but when a new account tries to access directly to http://saktolang.com/wp-signup.php

the user can still create a new blog..

what should I do?

and please don't recommend to download a plugin from wpmu dev.org..

I can't pay for a membership...