To de-authorise a computer, simply go to Account > Authorizations > Deauthorize This Computer
If you no longer have access to an old Mac or PC to de-authorise iTunes but have hit your five-computer limit, there is the nuclear option. Go to the aforementioned Apple ID Summary section in the iTunes Store and click Deauthorize All. This will de-authorise all of your computers. Apple states this option can only be used once per year, and you'll then need to re-authorise any computers you wish to use the relevant iTunes account's media with.
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