There are two "associations" possible for the computer.
1) Associated (Manage Devices): to play content purchased with your Apple ID (up to 10 computers and iOS devices). These can be removed in iTunes on your computer: Open iTunes Store, select Account in the Quick Links section. Find Manage Devices in the iTunes in the Cloud section.
2) Computer Authorizations - 5 computers maximum per account - there are two ways to remove an authorization: Deauthorize the computer using the iTunes software installed on that computer OR Deauthorize All computers (this is what you'd have to use if you no longer have access to the computer).
To deauthorize a single computer (when you have access to it) - open iTunes on that computer > Store drop-down menu > Deauthorize this computer
To deauthorize all computers (when you do not have access to one or more computers): Open iTunes on your computer. Open the iTunes Store, select Account in the Quick Links section, Select Deauthorize All in the Apple ID Summary section. NOTE: You can only Deauthorize All once a year. As long as you haven't reached your maximum of 5, it's probably best to wait on this until you really need to do it (if ever). Just remember to Deauthorize before you dispose of your computers from now on.
After Deauthorize All you have to Authorize the computers you still have. iTunes > Store drop-down menu > Authorize this computer...