Hello,
I recently tested a software named Dukto, I uninstalled (total uninstall option) using Install/Remove Software (Synaptic).
When reading my updates log after removal I noticed updates to the keys of the uninstalled program, as the keys remained.
In
this link Nitin Venkatesh answered a question about key removal, that I wished to share as it may save someone time searching it out.
To find the keys -
sudo apt-key list
Reading down the results I found the key for the project creator, 8 characters.
sudo apt-key del
After the word del 1 space then paste the 8 characters
Repeating
sudo apt-key list
Shows the key is removed.