First, and again, you are on solid ground with specific discussion topic focused communities.
Assuming a more general discussion, and or one where growth and or participation is desired, having strong norms in place immunizes the community against nefarious players.
When someone new shows up, the idea is to give them some rope. Let them self identify, and self select too.
When most of the community gets this, then the idea of raising both their cost and reducing their reward both pack a big punch.
You aren't wrong. What you put here does work, but it has a lot of friction, which may be undesirable.
See, a nefarious actor has a low risk and generally low cost to entry.
Why enter?
They also have a high reward potential, and it's that favorable ratio that consistently attracts them. Above someone mentions $10 as a barrier to entry.
$10 is cheap ass compared to the fun one can have! No joke. In my study of this, I've met many who would pay that easily.
If that friction, however high it needs to be, is desirable, fine. Raise the cost enough and the outcome will be a low noise, but also low churn, low growth community, generally speaking. A very compelling, but specific topic can alter that too.
Otherwise, the community, it's founders, moderators, others of status, influence, however that all is structured, only has control over the reward part, and they can have some control over the cost too.
See, the topic of discussion here is more general chatter. We have more focused things well on the way to being acceptable signal to noise. What we do t have is general community, say politics, or other broad topics in that same state.
It's that which I have focused on for a very long time. Personal interest.
Norms, and various discussion devices can deny nefarious actors joy, and can frankly make them regret their intent. At the same time, consistent calls to join the community, based on their own arguments framed in positive ways can work wonders.
One discussion norm is weighting of what one gets. When a clown calls you an ass, it's as laughable as it could be a basis for righteous indignation.
The vast majority of the time, righteous indignation is the response. "How dare you!" From there, the reward gets pretty good for the clown.
What happens when the response is laughable? One actually laughs, or does something basic like rate the shit?
I can tell you the reward on all that is much lower. When they realize that is a norm? Low joy, high pain potential, particularly when they figure out they really are seen as a clown, or ass.
All these things, and I'm leaving a lot out, as it would be a short book to model, get at reward.
Ask yourself, what does a reward look like?
Say it's attention, as another example.
Return that with endearment, and most often they will leave, or join and become a member with some genuine basis. They leave because the moment they are familiar, their behavior normalized, understood, it's boring. No fun. No joy.
They join, because they find out there really is a common basis and they didn't realize it. Often, an ask to join will actually work.
A few will just spew forth, and so raise their cost makes great sense.
Simple things, like say not allowing them 4 letter words, carry a very high, often funny cost that can always be removed on good behavior, expression of and demonstration of better intent.
The best is the community can see that, act accordingly and it's all public and largely transparent. Often, some members will step in to help. Often that works, amazingly, and when the basic norms behind it all are in place and solid.
Many other simple, subtle, always recoverable things can be used in tandem with strong community norms to inhibit noise, while promoting signal.
The best part about this kind of approach is it's well distributed. Model it to a few, they apply, model for a few more, and soon most active participants are largely immune.
An "infection" won't spread, and where it does, the cost tools are used to marginalize impact.
A lot really depends on community intent and whether friction to entry is desirable, or not.
The bigger problems cited here are most painful on general discussion, and most discussion is actually general.
Having more specifics out there is good, but not ultimately a solution for the general need and value humans find when they interact.
This is tribal. People seek tribes. A good tribe stands strong because it's members understand how to do that and why it matters.
An investment in those things can be very effective, and that was my point to make. The things you mention can also do that.
You aren't wrong. There are just cases where other ways may well deliver better results, or be better aligned to the community needs and goals.
Say those goals aren't to grow old and decline.