Some of them carry a little targeted advertising, others none at all. If you decide you don't want to continue, just follow the simple directions to unsubscribe. Much simpler than convincing a print publisher you really don't want the product. There are hundreds, maybe thousands, of online newsletters, on every conceivable subject, from investment advice and dog sledding to counted cross-stitch, genealogy, history, and astronomy. You can almost always do that with an email newsletter.
Standard practice is to provide a means for the readers to respond.
And I find that most editors and authors are happy to hear from readers who are polite and friendly, even if they differ. Good email connections have means to filter unwanted 'spam,' which is the online term for junk mail. You can follow simple directions to personalize your email so that you'll never even see most of it. I wish the Post Office could do that for me. Since the newsletters are the online equivalent of bulk mailing, you may have to do a little extra work to let the machine know they are exceptions to your 'No Junk Mail' rule, but the instructions for that are simple and easy, too.