I have added the "archive comments" feature that I mentionned this morning and earlier.
At the same time, I added a feature to easily delete several comments at a time if there is a flood/spam on the site.
I ask to the admins to be very careful with these new things... as I did not test that very much (though it seems to work).
About archiving comments, I think that it is better to wait few days before archiving comments on a page, to be sure that the off-topic discussion is ended and won't start again.
It is also better to try to select the comments to archive in a way that the remaining comments keep their meaning for visitors of the site.
Well, as there are not many pages concerned by this, I will maybe be the only one to use this function actually.