Yep. If you disable the back end they won’t be able to change the password. You want to disable it completely? How are people going to write blog posts?
Anyway, the password on signup is more than I have time for now. There is a premium plugin available on wpmudev.org http://premium.wpmudev.org/project/set-password-on-wordpress-mu-blog-creation that does what you want. I don’t know if it works with bp or not.