Hi there and welcome
You can only do this once a year, but to Deauthorize all computers you do the following
Open iTunes on a computerFrom the Store menu, select "View my Account..."Sign in with your Apple ID and password.Under "Computer Authorizations" select "Deauthorize All".You can then authorize each computer you still have, as you may require.
If you have any problems, or would like any additional information or assistance, please do not hesitate to let me know.
Else if you can take a second to rate my service by clicking one of the stars at the top of the screen then submit, that would be greatly appreciated.