If you are using ssh, you need to change the public/private key stored in the server account ~/.ssh
folder in order to use your own public key, and not the ssh key of the "resigned employee".
You need to generate your own set of ssh keys.
See this Gist for an example.
Check also if that old key is still offered by ssh-agent (that happens if your private key is passphrase protected): see "Having Trouble Switching Github accounts on terminal" for more.