Responsible email from a genuine supplier will have an unsubscribe link which must be honoured. Indeed in many regions, it is required by law for B2C email, (although not for B2B).
Unsubscribe.. or tell potential spammers you're there?
In the past, the advice has often been to ignore unsubscribe links as they simply tell the supplier that the email address is a good one so they will keep on sending. However for a consumer, such unsolicited email is no longer good business. It can be illegal and it can also quickly lead to poor sending reputation which really damages deliverability. Such advice is therefore a little out of date. You should unsubscribe and if the sender continues to send, register a formal complaint with your ISP. As a result, it is very rare these days for senders to continue sending after an unsubscribe. It no longer makes business sense.
So why not just mark it as spam and let your mail program automatically place future ones in your spam folder? This is certainly attractive as it is so easy, particularly if the unsubscribe process is made too difficult. However, this disguises a real problem. The spam filters in your mail program are almost always Bayesian classifiers. In other words, they learn the difference between spam and non-spam from your direction. They achieve this by measuring the relative frequencies of words and other indicators in emails you designate as spam and non-spam.
Built-in inbox-sorting; error-prone classification by humans required.
So what is the problem? Unfortunately, if you receive a high quality genuine email from a supplier you don't want to hear from any more, its characteristic Bayesian signature may be very close to emails from suppliers from whom you do want to hear. In other words, you are beginning to confuse your filters as they do not have much to go on. There is then an increasing danger of genuine emails disappearing automatically into trash.
This of course doesn't happen with poorly constructed spammy emails. Their signature is very different from genuine emails and your filters learn quickly and make very few mistakes.
On the whole then in this day and age, its better to unsubscribe.