You may have to read filter through a lot of extraneous information before you find the key words indicating why the message bounced. Carefully examine the error message and look for variants of the email address which may be subscribed to your list. Often people will have more than one version of their e-mail address in use. This just means that an intermediate host or the host itself will continue to try to deliver the mail that ListProc sent out a while ago. Many places continue to attempt sending bounce mail for a week.
Some sites try for two or more weeks.
If you are receiving a large volume of these messages, and, based on the messages, you expect to continue to receive them for another week or more, and you double-checked that the user is no longer on your list, you may contact the postmaster at the user's domain and ask if they could delete those messages from their mail queue so you stop receiving error messages. The subscriber may have an alias inside ListProc which is still sending them mail. You will be sent a list of the aliases for your list. You will find that if an internet provider has a transient error, the subscriber will be removed from your list. This can be very annoying at times when subscribers discover they've been deleted from a list against their wished.