Thanks for posting, I am sorry to hear of your experience. Once information is part of these contacts the system is designed to update rather than delete if the field is blank. Can you tell me more about the information you don't want? While importing you will want to choose "Do Not Import" as the label for the fields you don't want included moving forward.
Are the contacts all in lists you want them in as it is now? Ideally would you like to delete the information from each contact or just leave it and make sure that you are updating the fields you want to use?