How to stop adding IP from EC2 to known_hosts for ssh?

Try this:

ssh -q -oUserKnownHostsFile=/dev/null -oStrictHostKeyChecking=no -i $MYKEY $MYUSERNAME@$MYIP $MYCOMMAND

You can also do this in your config file:

Host *.amazonaws.com
  User root
  StrictHostKeyChecking no
  UserKnownHostsFile /dev/null
  LogLevel QUIET

This is done to prevent Man in the Middle attacks. Disabling it would disable basic functionality of the ssh tools.

You may want to keep a copy of your .ssh/known_hosts file without the entries and replace it when you are done.