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Convert Forms does not support preventing a specific field from saving in the database. However, you can remove the field value from the database after the submission.
Setup
To remove a field value from the database, place the following code into the PHP Scripts -> After Form Submission option in the form builder.
\ConvertForms\Api::updateSubmissionField($submission->id, 'FIELD_NAME_HERE');
Replace FIELD_NAME_HERE with the name of the field you would like to remove from the database