/usr/nsh/sbin/agentctl stop (replace /usr/nsh with your actual install directory).
You can also start it with
/usr/nsh/sbin/agentctl start (I believe you might want to give agentctl some additional flags like -r and/or -b).
You should be able to use stopsrc to do this, but it doesn't look like the agent installer actually creates the "service" using mkssys. If this was the case, you would be able to use stopsrc -g tcpip -o rscd to stop it.
I will file a bug/enhancement request for this. Outside of agentctl, which Sean describes in his reply, there isn't a good way to do this except by getting the PID and killing it.