Ability Define Unique Field Options

I'd like to define which options a user can select using the "List Selection" and make each option unique. Therefore, if the options are A, B, C, D. Only one option can be assigned to each user. I want to do this to allow users to select pre-generated usernames for their children (which is a custom object in Ontraport). This would be a helpful work around since we can't custom object data to and from Ontraport without 3rd party integrations/coding.