When you're fighting spam, you reach out to a lot of different people. If you received spam and you're savvy enough, you send it to the ISP from whence it originated, and ask them to take action to make the spam stop. If you work for an ESP or a blacklist group, you might reach out to the sender and ask them to prove that this person opted-in, with details, in order to resolve the "he said, she said" situation inherent to most spam complaints.
Since you're dealing with a lot of different people, with varying levels of understanding (and interest), it's easy for the message to get misinterpreted, no matter how hard you try. For whatever reason, the person responding to you answers the wrong question. A client (sender) might reply and say "the complainer has been unsubscribed." (Great, but that wasn't why I contacted you.) A snarky "Why don't you just hit delete" response from an ISP, disappointing that it is, isn't entirely unheard of. (Great, but I was writing to you to let you know that you have somebody on your network violating your access policy, and possibly violating the law. You don't care?)
Here's a prime example. My friend Steve forwarded me this reply he received from ISP WorldStream, located in the Netherlands: "I'm the supervisor of the abuse group, you probably signed yourself up to a website and now you can sign off the mailinglist through the unsubscribe link that is less work then sending us an email. Have a nice day!"
Great, except that it entirely misses the point. Steve contacted you, told you that no, I didn't sign up for this -- it was unsolicited. That's what a spam report is-- it is the recipient telling you that this was NOT something that I signed up for. To reply with "eh, you probably asked for it" is disappointing. It may not be entirely unexpected (to me), but it is most definitely a lazy response. It is not a consumer friendly response.
If, instead, you don't want to be like Goofus, you'd better provide a better response, and handle this situation differently than WorldStream did. Reply to the complaint -- tell the complainant that you've received their complaint, and that you will keep an eye out for additional complaints, ensure that the mail stops, and if an access or permission policy violation is found, further action will be taken. Then, actually do that. Contact your client. Ask them how people sign up for these lists. Ask for details showing the date, time, and source IP address of the relevant registration for this specific complaint. Does the signup process seem legitimate? Are you sure this isn't a purchased list? Are there other complaints? Sometimes a single complaint isn't enough, so you'll have to wait and see if additional complaints are received before you can gain a better understanding of what might be happening.
But whatever you do, don't provide lazy or useless responses. A very long standing goal of anti-spam blacklists is to deal with ISPs who fail to take action against spammers. They start by blacklisting the spammer themselves, and then if the ISP is complicit or otherwise unwilling to address the spam issue, it's not uncommon for the blacklist to escalate the issue by blacklisting some or all of the provider's IP addresses.
I certainly see this in the ESP world. Recently, a very big blacklist blocked all 3000+ IP addresses of an ESP over an issue stemming from one single client's mailings. And that wasn't the first time I've seen that kind of thing. On other occasions I've seen a blacklist level listings of 256 IP addresses (one /24) or 512 IP addresses (a /23 range) to proclaim the blacklist's frustration that the sending network's abuse desk is not handling a spam issue to the blacklist's satisfaction. That pressure is specifically applied (I suspect) to force the sending network to deal with the issue. It causes other clients' mail to get blocked, making them very unhappy, and they're going to take that unhappiness out on you, the service provider, not on the blacklist operator.
Some people think this is very unfair. Fair or not, it's going to continue to happen. If you don't want an issue to grow to that level, you need to treat spam complaints seriously. The lazy path leads you to a place where very bad things can happen.