The system will not allow you to delete lists that have been sent to in the last 30 days. While you can technically delete lists after the 30 days has lapsed, we strongly recommend against doing so. When you delete a list you lose all the records associated with that list, such as sending statistics and subscriber preferences.

If you delete someone who has opted-out of communications from you, you risk re-importing them at a later date and sending to someone who might mark you as spam for doing so.