If you render a scene that has Skip License Check enabled, you’ll get the Arnold watermark. One way to make sure this doesn’t happen is to turn off Skip License Check from the command line:
render -r arnold -ai:slc off some_scene.mb
kick -set options.skip_license_check off -dp -dw some.ass
You might also want to enable Abort on License Fail (-ai:alf for render, -set options.abort_on_license_fail for kick).