I didn't know that users can change their username! May be cool feature for some but I don't want to let users change their username and email.
Any plugin or tutorial for this task?

In the CB>Configuration>User Profile

Set "Username" to "NO"

Set to "Yes" to allow the username to be changed. If set to "No" then the username will not be editable after registration.

Thanks. Any idea about setting email unchangable?

I too would like to know about making the email address unchangeable by the user, or atleast require that they need to go through email confirmation again. Any ideas?

Yes this is a big issue for me on my site with 3500 users

I am currently looking for a program that can check validity of email adresses using the smtp protocol, without sending an actual email.. (i dont wish to parse all the bounced emails).. and then send email to all users warning them that their accounts are going to be delete.. then auto ban everyone that no longer has a valid email after a month..

i know such a email checker exists in the underground scene..

Something like this is essential. Or CB team can realease a patch which will let admin decide like "username" whether to keep it changeable or not.