BuddyPress.org

– Is it possible as admin to change their username in the back office?

No, WordPress does not allow it. Usernames must be unique.

– Why the dot/period or special characters are allow in the username during the registration? and how i can block this?

There are various methods to restrict the characters allowed in a username when registering. Here’s one example:
http://wordpress.stackexchange.com/questions/124697/limit-username-to-specific-characters-a-z-and-0-9