"A cease and desist letter can be sent to the person who is harassing you to demand that they immediately stop their behavior, which may include unwanted communications, stalking, or threats." and "Harassment is generally defined as unwanted, unwelcome, and uninvited behavior which annoys, threatens, intimidates"

I've always wondered if that would apply to newsletters who ignore/refuse my opt-out request. "unwanted communications" and "uninvited behavior which annoys" is pretty much how I feel when companies keep sending me their newsletter after I told them to stop.