hi DJ
Open itunes
On the top left of the screen you will see menu choices [file edit view controls store help]
click on store and from the submenu choose "view account (your account name)"
You will be asked for your username and password, enter these and click on [view account] button
Your account page will now open (account information)
On this page, there are several sections for different information categories
the top section is "Apple ID Summary"
the next down is "Itunes in the cloud" (this is the section you are interested in)
In this section there are two subsections, Manage devices and Itunes Match.
to the right of the subsection (Manage devices) there is a hyperlink "Manage devices >"
Click on this hyperlink and you will be taken to the Manage devices page where all your registered devices are listed. To the right of each device is a [Remove] button.
click on the remove button next to the device you wish to de-register and it will be unasociated with your apple ID
Hope this is clear
Regards
Stylo9