Hi David,
You likely figured out a solution by this point, but for the sake of completeness, your approach would likely have worked correctly if you had used just DOMAIN != ‘MQ’ in your When clause. This wouldn’t have exactly disabled the rule (it would still execute), but since the DOMAIN variable would have always equaled MQ in the MQ dataset, the rule would never have failed.
The only way to have completely disabled the rule for that domain would have been to create a custom configuration that included an additional ItemGroupDef for your custom domain, along with imports for the rules you did still want to execute as per the configuration guide.
Regards,
Tim