Rest API: How to clear custom field value (multiple values)

Hello!

I recently ran into an issue using the REST API.
I'm attempting to update a custom field of the type enum[*].
I used to do this by first updating the custom field to it's empty value and then sending an update for each selected field. However, I now have a field which cannot be empty so the empty value (default) is part of the list and my trick doesn't work.
How can one deselect items from such an enumeration trough the REST API?

Thanks a lot!