Next to that, extract what you really have to. There is also a Reminder appended to every posting, reminding people of what the current topics are. There's no need to include that in your reply. People get it every single mail and still they make mistakes. Sometimes lists get spammed.
Someone subscribes to a lot of lists, sends an advertisement through it and unsubscribes.
A mail to the postmaster of the domain of the offender is probably the best thing to do if you need to act upon it. It's not too hard to fake an email address, so someone could hack into the account of someone else and spam, unknown to the real owner of that account. As mentioned before, there is an additional web site to this list. Although the discussion list is purely for scientific means only, on the web-site you might expect a few commercial items.