You are on a mailing list and believe that a certain topic that is being discussed is actually off-topic for the mailing list and should be discussed somewhere else. Do you:
Mailing List Etiquette
Post a lengthy response giving your detailed opinion about the topic which concludes by telling everyone that they should henceforth talk about this in another more appropriate (and specific) place, since the subject is off-topic for the list.
Post a brief note to let everyone know that they can find your response on this topic (which you believe to be off-topic for the mailing list) in a particular, more appropriate location where everyone might want to continue the discussion.
Hit the delete key and ignore the whole thing, hoping that it will go away.