Rigorous intellectual debate has it's place. You have to definitively disprove something at the start. But at a certain point in time, giving them any more spotlight does more harm than good. There is a certain portion of the population that will be swayed into ridiculous viewpoints no matter how thoroughly they have been destroyed in a debate, no matter how much evidence has been piled in front of them. Rigorous debate of flat earthers rarely convinces the flat earthers they are wrong, and doing so in a public setting provides them more opportunity to spread their misinformation. We would gain nothing of import by putting a bunch of flat earther's on national television and debating them.
Nor would you gain anything by platforming a KKK member and debating them on stage today. There's no advantage to be earned by doing so. But, at the time, when they were at the height of their power and had many people believing in them? Certainly. They had a way to preach their message regardless, the reach to spread the information to a large audience. Being able to argue against them and destroy their message was important.
But cancelling them was also part of the process. Boycotts were a SIGNIFICANT part of the effort in defeating the clan. You can find many, many, many historical references to them. Here's a small sample: https://thecatholicnewsarchive.org/?a=d&d=TCT19221207-01.2.6... https://www.jstor.org/stable/27502105 https://academic.oup.com/chicago-scholarship-online/book/214... https://digitalcommons.liberty.edu/cgi/viewcontent.cgi?artic...
The American Unity League in particular spent a large amount of time and effort organizing boycotts and other methods of cancelling the KKK. It was important and it was effective.
If the context is a vacuum that might be fine, maybe.
In reality the US is hyper fragile intellectually and it has gotten drastically worse over the past 10-15 years. The way that fragility is being managed is through silencing and cancellation instead of through intellectual strengthening. Younger people in the US are entirely incapable of discussing difficult ideas emotionally, they're weak. Today the US would try to defeat the KKK via cancellation, which doesn't actually work; yesterday the KKK - which was a huge movement at one time, and has almost no power today - was defeated in the public square head-on, not by cowering or cancelling. The people that intellectually fought the KKK at the height of its power would ridicule today's incredible mental weakness; such weakness that someone as trivial as Trump has to be cancelled in order to deal with him. If people today weren't so intellectually weak, they could counter a Trump quite easily. Trump is absolutely nothing compared to what was dealt with in prior generations.
You defeat bad ideology through rigorous intellectual conflict in the public square. It's messy, difficult and it can be violent - so what. Anything else and the bad will fester under the rugs where it has been swept, and you risk it getting far worse. There are far worse things than Trump and they're barrelling toward the US right now (DeSantis), that wasn't stopped by silencing Trump; it only gets stopped through exactly what I said - you have to smash the ideas in the public square, your ideas have to win. Or else. The far right will eventually produce the next version of Nixon, and he'll wield far greater executive power compared to what Richard Nixon had. Trump isn't that, he's a carnival barker at best; a big part of the left is too irrational and obsessed to recognize the difference.
The US is lucky it was Trump. He's a de facto clown show. The US is increasingly close to being primed for real authoritarianism, the levers are there.