Hi suiauthon!
You can't de-authorize a machine you don't own anymore. When you reach those 5 authorizations, you'll be able to de-authorize all your computer. To do so, please follow the instructions above:
1. Click iTunes Store in the Source panel.
2. If you're not signed in to the store, click the Account button, then enter your account name and password.
3. Click the Account button again (your ID appears on the button), enter your password, and then click View Account.
4. In the Account Information window, click "Deauthorize All".
After removing them all, just authorize your current computer. Note that you con only use this feature once a year!