Sometimes 'unsubscribing' does seem to reduce spam activity
There is one particular email sending place that has for some time been the most prolific source of sending attempts to some of my old addresses. I put them in IP-level blocks literally years ago (which means that nothing they've sent me for years has been delivered) and ever since then I could count on their relatively small netblock to be either the very top blocked traffic source or at least one of them. They have in the past been SBL-listed, although they may not be right now, and there are plenty of reports all over the net of bad behavior. If you had asked me to pick someone who would never, ever stop spamming me they would have been at the top of the list.
When I started my sinkhole SMTP server experiment, of course they were one of the people who sent me email basically immediately (within a few minutes of the service starting). So I tried out their unsubscribe link to see what would happen. To my surprise I have gotten no email from them in the 20 days or so since then, when normally I would expect to be awash in it.
This is not a definitive test by any means, for many reasons (including that it's early yet). But it's at least an interesting and surprising result for me. Given that these people have ignored what is probably a half a decade worth of bounces, they were about the last people I'd have expected an unsubscribe to work on.
(I think I'm going to avoid speculating about why they might totally ignore years of bounces and non-delivery reports yet respond apparently instantly to an unsubscribe attempt, mostly because it would all just be complete speculation and guesswork.)