How do I delete ALL SSH known hosts?
First of all you should remember to verify ssh key finger prints when connecting to a remote computer for the first time, to avoid MITM attacks.
Having said that, first making a backup, and then removing all previous ssh known hosts is a matter of doing this on your local computer :
cp -av ~/.ssh/known_hosts ~/.ssh/known_hosts-old
rm ~/.ssh/known_hosts
The known_hosts file will be created again after you completely initiated the first new ssh session.