You normally should be able to use:
sudo apt-key del <KEY_ID>
But if that fails you can search for the corresponding key file in /etc/apt/trusted.gpg.d
and delete the two files for it there.
Here an example, lets say you have installed the tor browser from ppa:webupd8team. After uninstalling this package and removing the source files for it from /etc/apt/sources.list.d/
you still have the two keys in /etc/apt/trusted.gpg.d
named
-rw-r--r-- 1 root root 507 Okt 21 23:36 webupd8team_ubuntu_tor-browser.gpg
-rw-r--r-- 1 root root 0 Okt 21 23:36 webupd8team_ubuntu_tor-browser.gpg~
Now remove both keys by:
sudo rm /etc/apt/trusted.gpg.d/webupd8team_ubuntu_tor-browser*
And you should be done with them. you can verify this by opening Software&Updates and check on the 'Other Software' tab. They key should be gone. you can find the proper file name as well with apt-key list
the output for this key I used as example looks like this
/etc/apt/trusted.gpg.d/webupd8team_ubuntu_tor-browser.gpg
--------------------------------------------------
pub rsa1024 2010-05-04 [SC]
7B2C 3B08 89BF 5709 A105 D03A C251 8248 EEA1 4886
uid [ unknown] Launchpad VLC
You see the name of the key file is stated above the key itself.