The watermark will appear on renders if Arnold can't find a license.

Only those renders run by batch (BackBurner) are watermarked without a license. Renders run from inside 3ds Max are not watermarked.

Get a license

To remove the watermark a license server needs to be installed and valid license running. See the Licensing page for more information.

I've got a license but the watermarks are still there

If the watermarks are still there, go to the Render Setup, and in the System tab, verify Skip License Check is disabled. With this setting enabled Arnold will always render your image with a watermark even if you have a valid license.

But I don't want any watermarks ever

When running on a farm, you sometimes prefer the render to fail entirely when no license is available than have watermarked images. Tick the Abort On License Fail option to enable this behavior.