How do you delete the data in Marketo Custom Object records, or in specific fields on the custom object if the the custom objects have already been created and approved?
Marketo Custom Objects are not accessible via the User-Interface. API calls must be utilized in order to delete this record data.
You can delete the records via the 'Delete Custom Objects' call
If you are interested in extracting any of the existing data, you can use the following GET call. You will need to make a 'Get Custom Objects' call
Here is an additional resource that may be useful and relevant