The only way to make the custom field not populate is if it is not set/blank on the customer record. In turn, there's a preference to not update the customer records, and also - even when that is not enabled - I see a prompt asking me if I'd like to when I save a sale. If I say no, then the field is not set on the customer record.

To set the preference, go to Edit | Preferences | General | Company.  Then enable the setting "Never update name information when saving transactions."