I see how such a feature would be useful in preventing users from bickering, but, as stated above, allowing the thread creator to control a hypothetical thread blacklist would very likely lead to the stifling of opinions, particularly on the Consensus Track board. A past example of this would be the infamous "Consensus against Jimeee" thread from 2014, in which Jimeee was not allowed to defend himself in his own demotion CT because of a "conflict of interest." This was obviously quite unfair for Jimeee, and it ultimately ended in his resignation.
Also note that this was enforced by an administrator, Ghost Anubis, so it's not like admins are necessarily going to be totally reasonable. You could perhaps argue that since sysops already have the power to block users, a thread-specific ban is no different. However, such a feature would likely encourage administrators to impose user thread bans preemptively or prematurely, whereas a block is an entirely reactive tool. As with Jimeee's CT, this won't always end well.
I'm also imagining some technical/logistical difficulties with this sort of feature. If it were to be built into the Forums it would require a certain amount of new JavaScript, and the Forums aren't exactly the most bug-free feature as it is. We also wouldn't be able to port this type of functionality over to Discussions after the merge. We would therefore have to track the thread blacklists manually; while this might not be too difficult for a small number of threads, it would be nigh impossible for 22,000.
One alternative would be to create some sort of ban on user interaction between two individuals in which they are free to do as they please except for talk to the other person, which would result in a block. This would be easier for the administration to manage, although it could still result in the same issue as we saw on Jimeee's CT. I believe this is in practice on the UESP and some other wikis, but I would have to look into how successful it has been before saying anything decisive about it.