There are situation in which you want to start from a clean slate on Mac OS X after mess up with a new Mac machine out of the box. Removing accounts from System Preferences is very easy, but doesn't help in this situation. Below is the version from Single User mode
1. Boot in single user mode (hold Command-S during boot).2. Execute the following lines
mount -uw /
rm -R /Library/Preferences/
rm -R /Users/username/
rm /var/db/dslocal/nodes/Default/users/username.plist
rm /var/db/.AppleSetupDone
3. Shutdown