how to deauthorize a computer I no longer have

This is explained in the support document Authorize or deauthorize your computer for iTunes Store purchases - Apple Support

Scroll down to How to authorize or deauthorize your PC

If you need to deauthorize a computer that you don't have access to.

To deauthorize a computer that you don't have access to, you have to deauthorize all of your computers. Then authorize each computer that you have again. You can deauthorize all computers only once a year.

1. Open iTunes.
2. Choose Account > View My Account.
3. Sign in with your Apple ID and password. Forgot your Apple ID?
4. At the top of your Account Information page, go to the Apple ID Summary section.
5. Next to Computer Authorizations, click Deauthorize All.
6. Authorize each computer that you use again.