If you are creating new users using Bulk Task, CSV file, and would like to avoid to create a new one based on the attribute, example customFieldXX="holiday", you can use the Policy XPress.
Identity Manager 14.x
You can use the Policy XPress, see image below, and set the Type to "Throw an Exception"
Note: Run several tests in your Test environment before implementing it on the Production environment.
More information about Policy XPress see the link below: