I really think that GitHub needs better tools to deal with this, this is very frustrating - the only options right now are deleting a ton of comments or locking it preventing all discussion. Prompting people with a modal informing them what comments are inappropriate would go a long way.
In the past you would create a few mailing lists; I think sourceforge automatically did that for you too.
They should have locked the thread or use a wiki page for this. A wiki page is less likely to be edited to add this kind of comment or at least it would be easier to clean up with a bot.
