You can't change the rules after the fact for the same reason why you can't self-mod a thread after you created it as it wouldn't be fair. As shorena said, just lock the thread and create a new one (and ideally self-mod it if you want to inact your own rules).
So I can't do what I did here:
https://bitcointalk.org/index.php?topic=1423718.msg14425704#msg14425704?-snip-
New local rule: franky1 is banned from this thread from this post onwards.
Nope.
Why not? It isn't retroactive.
The wording of the rule does not allow it.
26. Local thread rules, if stated properly when the thread was started, specific enough and don't conflict with the forum rules, have to be followed.[e]
In the example there is also a quote by theymos that this was mostly replaced by self mod threads. I would think the same applies. If you want a local rule (or a self moderated thread) you have to do so when you create the thread.