Delete field in Eloqua Custom Data object

Hello

We have created a CDO with few fields. We bulk imported data using SFDC App. Now when we are trying to delete one field, which is unwanted, then it is throwing below error:

Alert! -This is referenced by the following items:

Please Edit or Delete all dependency references listed below to allow the item to be deleted. If the Edit and Delete buttons are greyed out, you should remove the dependency in the original reference. Note that depending on your user rights, you may see only a generic message or you may not be able to view or delete the dependency in the reference, so you may not be able to delete this item. Talk to your Eloqua Administrator or Solutions Manager for more information.