Hopw do I deauthorize a device when I don't have it anymore

Deauthorize all computers - https://support.apple.com/HT201251#deauthorize

"If you sold or gave away a computer that's used one of your 5 authorizations, you need to choose Deauthorize All on the Account Information page. Then authorize each computer that you still use."

If you mean removing from your account', you can use the 'Manage you Apple ID' page https://appleid.apple.com/account/manage

and in the 'Devices' section you can click on the name of the device (in blue) and 'remove from account'

If you just want to sign out, you can use the iCloud settings: https://www.icloud.com/settings/

select the device, click on the blue cross

But if the device reconnects to internet it will reappear in that list