iTunes: how to "deauthorise" computers I no longer have?

Currently linked to my iTunes account are 3 computers that are “authorised” (you can only have up to 5) – the problem is that I only use one of these “computers”, the other two “computers” are just from seperate times I had to reinstall my operating system. Is there a way to deauthorise these “computers”, considering that they don’t exist anymore?

I have tried following guides on the web which say to go to the iTunes Store, then to My Account, and click on something like Deauthorise All Computers, but this option isn’t available for some reason? Could it be because I’m using an Australian account?