Using terminal, go to /Users/Username/.ssh
Open the known_hosts file with a text editor of your choice
Each line here is a host that has been permenantly registered on your machine.
Delete the line that belongs to the unnecessary host.
Tip: In VIM, dd is the shortcut to delete the entire line.
PS. Life tip: To exit VIM, make sure you are in command mode by clicking ESC [escape] button on your keyboard/touch bar and type the following: