You can only delete an email list if it has never been used for a campaign. To determine if it has been used in a campaign or not, go to the Contacts Page in the Email Marketing Module. Select the list you would like to delete. If delete shows in the bottom left corner, you can click on delete to open a new window to confirm the deletion.
If delete is not showing, you can instead make the list Inactive by clicking on edit in the bottom right corner. Uncheck the box "Active" to mark the list Inactive and click Save.