The Mac OS X operating system includes a tool called Directory Utility that lets administrators add Apple computers to domain networks. By putting Macs onto the same domain, you can control group authentication and file sharing. If you no longer want to use your Mac with a specific domain, an administrator can log on to the computer and remove it from the domain using Directory Utility.
Click on the "Finder" icon in the dock to open a new Finder window.
Highlight your internal hard drive from the "Devices" list on the left side of the window.
Open the "System" and "Library" folders.
Open the "CoreServices" folder and then double-click on the "Directory Utility" icon.
Go to the "Services" tab at the top of the window.
Click on the lock symbol and enter your administrative username and password.
Highlight the item labeled "Active Directory" and then click on the "Edit" button.
Click on the "Unbind" button and enter your administrative username and password again.
Press "Apply" to save the settings and remove the computer from the domain.