They are often obscene and play no part in any beneficial role to humanity. Unlike other forms of parody or expression they do not encapsulate the views of a person but instead simply are spam of the highest order. It is not a form of any plausible, intelligible communication whatsoever.
Even though more often then not the content of these chain emails are morbid there is no evidence that the circumstances in these letters actually happen. More often then not they just say to forward and no further harm will be done. There are more malevolent letters out there but it is up to the readers discretion what they do. I have read many of these letters and nothing has happened. To use these letters as an excuse for doing bad things is ludicrous. People are responsible for what they do not an letter online. And if you can't handle it then you probably shouldn't be online.
Well, unless you want actual people sitting 24/7 watching numerous videos' chats looking to see if someone has posted a chain message. Reporting them is the most that can happen at the moment. I don't know how you would be able to ban them completely. Maybe I'm missing something here?
At least in the United States and in other countries that have documents similar to our constitution, chain letters should NOT be banned because of our freedonbof speech. Although these chain letters do tend to be very ludicrous, false and attention-seeking, we have the right to comment on social media with chain letters. They are irksome, however.