Russ Freeman
If there were a mechanism for removing a post which is at best tangentially related to the main stem to somewhere else for that conversation to continue elsewhere, I submit that would be a gain, and render removal of a post a less nuclear option - thus tactically more useful to mods.
Computers are said to be good for automatically doing a set of things each if which could be done by a person, and being more reliable at it.
The breadcrumb trail each way is the slightly special sauce .
It would also be possible to display stubs differently from stems, in recent threads. Either with a delay of a day/week/month or not at all - the stub accessed only via the portal post it hangs from - or as subheadings from the stem thread, which I suspect you'd regard as more work than it is worth.
The OP for the new stub is the post which is bud/replied to.
It should have a link back to the earlier thread, possibly to the top of that, possibly to the "This post has been pruned and replanted because it is an off-topic-shoot, follow this to read or reply, please stay on topic here".
You probably didn't want your words deleted and a snotty remark about not writing like that. Which do you prefer less (a question people demonstrably often not good at answering, ah well)
Seriously: the librarian puts your book on a different shelf they think it belongs on Vs the librarian burns your book for being Bad.
One you can be cool about, the other you hit 451°F, much good that it does you.
To alter people's behaviour from actions you prefer they eschew to actions you regard as virtuous, or less lacking in virtue, you have limited classes of actions, none of the subtle ones being entirely proof against the person determined to pursue the opposite of virtue.
(Against those, taking off and nuking them from orbit must be tempting, but does cause Riples)
1. Ask them not to. You can nag everyone every month, every thread, every post, or ask them to not do it again.
You can tell them.
You can remove their access.
Alternatively and separately and concurrently, you can give them an easy way to do the thing you prefer or hate less or which leads to less nuisance if not to greater utility: a button which buds this fascinating rejoinder into another sub thread. (With a copy, like the default complete quote you offer for replies but the attributes attached)
A little finesse would steer all replies gently that way as well.
So "prevent" is hard, but encourage is less hard.
If the mod repotted the comment as a new sub thread, then it won't be there for people to reply to, a stronger option than a response as a bud. The mod, perhaps even the maker of that nodal comment, might come later to bury it.
I think you have tools and an attitude to harassment already, I'd suggest having provided points to branch the mods would have less of their time spent trying to stop derailments on the main line, and could ... have a nice cup of tea, or seek out the unrighteous in general. It would be quite a subtle sort of harassment, I think, so much so I wonder if it would even work.