Samba can typically be stopped using SSH to issue the following command:
service samba stop
or
/etc/rc2.d/S20samba stop
Problem is that upon next restart or power on the Samba service may be restarted. Because you are using the v4.x firmware, one may able to modify the /etc/rc2.d/S98user-start
file to include one of the above commands to stop Samba upon My Cloud v4.x startup/power on.