How can I delete one of my devices from the "lists of known hosts (ECDSA)" after connecting to it with SSH? How can I delete one of my devices from the "lists of known hosts (ECDSA)" after connecting to it with SSH on Sierra ?
You can delete a device entry by navigating to ~/.ssh/known_hosts and remove the respective line:

In the Finder hit cmdshiftG, enter ~/.ssh/ and hit Go. Open the file known_hosts with a text editor and remove the line with your device's host name or IP-address and the ecdsa key hint (e.g. ecdsa-sha2-nistp25).

Open Terminal and enter nano ~/.ssh/known_hosts. Move the cursor to the respective line and hit ctrlK to remove the line. To save the modified file and exit nano hit ctrlO and ctrlX



The known_hosts file looks like this:
ip-address (or host name) key_hint key 

→  one line per ip-address/host!
Example:
host.example.com ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDWn...
192.168.0.7 ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDEIb...
192.168.2.7 ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDEIb...
host3 ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDWn...
github.com ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAq2A7...
host2.example.com ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHA...

Opening the file in an editor like TextEdit the lines are probably wrapped:

To remove a host (in the example github.com), choose ip (or host name), key hint and key and delete it.
Editing ~/.ssh/known_hosts removes the host for the actual user only.