Hi Anthony,
Thanks a lot for your answer!
We're not trying to delete the user list: what we're trying to do is to remove the user list assignmnent from an adgroup. Hence, we cannot close the user list because it's also assigned to another adgroups.
We need to delete the user list assignment from a deleted adgroup. The assignment status in the AdWords console is shown as "Ad Group Removed". When we try to delete this assignment using the AdWords console, we get an error message:

Is there any way to remove the user list assignment that points to a deleted adgroup?
Thanks again!!