If a specific user posted many "offensive" posts in a single thread, downvoting these should be acceptable; subsequently downvoting many posts by the same user in other threads (perhaps as a form of punishment) should be unacceptable.

That is precisely my position. I just want everyone to be clear: you know very well when you go on a downvoting campaign against a user; and it will be very clear to us as well. There is no cover of darkness.

Thank you for your good suggestions!