You can kill ssh-agent by running:
ssh-agent -k
!NB, -k argument works only if $SSH_AGENT_PID environment variable is set. eval "$(ssh-agent -s)" sets the variable, but there’re also other methods to start the agent without setting the environment variable. E.g., connecting to a remote server implicitly starts the agent without setting the variable. In this case, you need to use pidof ssh-agent or pgrep ssh-agent to find the ssh-agent PID and then kill it.