When someone repeats an argument that has been proven false /badly argued many times before, but keeps repeating it in hopes of drowning out opposition or derailing a thread. Yet not disruptive enough to get banned on forums, as it wraps itself in non-hostile, nicely written sentences.
How exactly do moderators deal with this kind of behaviour?
Repeatedly using a blatantly false assertion is called the Big Lie (https://en.wikipedia.org/wiki/Big_lie). The derailing/zone flooding aspect, as @NOT_RICK@lemmy.world stated, is a Gish Gallop (https://en.wikipedia.org/wiki/Gish_gallop). There might be a term for using these together, but I am not aware of of what it might be.
Neither of these really apply?
The big lie refers to a very big lie, but OP is talking about something subtle.
A gish gallop refers to many lies, such that an opponent cant refute all of them, but OP is talking about one lie.