You can remove Auto List behaviour at anytime. Removing the behaviour will not remove the current Cards from your List. This ensures there is no loss of data, and allows you to edit the List as you desire.
Once you have removed your Auto List behaviour it will behave as a "normal" List, meaning that you will be able to manually add or remove Cards.
1. Open your Auto List.
2. Click the lightening icon on the toolbar.
The Settings dialog will open.
3. Click the bin next to your Auto List behaviour.
The Auto List behaviour field will state "None".
4. Click Save.
Your List will look the same, and the Cards will remain, but you will now have access to the "Add Card" and "Remove Card" buttons. The lightening icon will also be gone from the toolbar.
Also see "How do I Edit my Auto List Behaviour?".