This Skype bug was noticed in September 2015, where Skype users (myself included) could not delete a Contact List, formerly know as Contact Groups, as noted in the Skype Community:
You know you had deleted a Skype contact list the other day, but when you signed in to Skype, the list was there again. What happened?
The solution was found in a thread, How to delete Skype lists that were originally Messenger Groups? that called these Skype contact lists “zombie lists” and here’s the steps involved to permanently deleting these contact lists:
- Go to People app website <people.live.com> and sign in to your Microsoft account (same account that you use for Skype).
- Make sure you are signed in to Skype Desktop as well.
- In the People site, on the left side, find each Skype list (Messenger group; hereafter called “group”) you want to remove.
- For ease of searching, try disabling some of the contact types (such as Outlook, Skype, Messenger) in the Settings menu until only the groups you want to remove are mostly shown. The menu appears when you click the gear icon near your name on top right.
- Click on the group, so the contacts (if any) are listed on right.
- Click on Delete in the top bar, then click on Delete in the popup to confirm.
- Note that the deleted group may still appear in the left list – this is a bug of People app. If you reload the site, it will go away.
- Go to Skype Desktop’s Contacts pane, and open the group.
- Click on the Delete icon near the group name, and click Delete button to confirm.
- Repeat the two delete actions (steps 3 to 7) for each group you want to remove.