The AdWordsUserListService can be used to manage and create user lists. However, it is currently not possible to Remove the complete user list using the
AdWordsUserListService.mutate() as the REMOVE operator is not supported. However, you will be able to use the
mutateMembers operation to update or remove the user list members. Based on the code snippet shared, it looks like you're using the REMOVE operator to remove the entire user list using the mutateMembers operation which is not possible to do so.