Apple - Turn off the SSH Agent

unset the environment variable $SSH_AUTH_SOCK. In bash, from where you run the vagrant commands, simply:

export SSH_AUTH_SOCK=""

launchctl stop com.openssh.ssh-agent

eval $(ssh-agent -k) kills the agent in the current terminal.

It runs these commands

unset SSH_AUTH_SOCK;
unset SSH_AGENT_PID;
echo Agent pid 89141 killed;