I'm part of several communities that are relatively decentralized. For example, anyone can host a contra dance, rationality meetup, or effective altruism dinner. Some have central organizations (contra has CDSS, EA has CEA) but their influence is mostly informal. This structure has some benefits (lower overhead, more robust) but one drawback is in handling bad behavior. If several people reported very bad experiences with someone at my local dance we'd kick them out, but that wouldn't keep them from harming others at, say, any of the hundreds of other events run by other organizations.

I have seen cases, though, where someone was fully removed from a decentralized community. Looking at why these cases succeeded and others failed, I think it took:

Clear misbehavior, in a way that nearly everyone would agree if they looked into it.
Detailed public accusations, so people can look into it if they doubt the consensus.

The combination of these means that you can have an initial burst of 'drama' in which lots of people learn about the situation and agree that the person should be kicked out, and then this can be maintained whenever they show up again. For example:

2016: Gleb Tsipursky from the EA community, for a range of shady things and a pattern of apologizing and then continuing (details).
2017: Jordy Williams from contra dance, after accusations of grooming and rape (details).
2018: Brent Dill, from the rationality community after accusations of sexual abuse, gaslighting, and more (details).

Unfortunately this approach relies on people making public accusations, which is really hard. We should support people when they do and recognize their bravery, but people will often have valid reasons why they won't: fear of retaliation, unwilling to have that level of public scrutiny, risk of legal action. In those cases it's still possible to make some progress privately, and we definitely need to try, but you keep bumping into the limitations of decentralization and defamation law.

To clarify why I think (1) and (2) are key for community-wide exclusion, however, let's look at two cases where this has been tried but was only partially successful. Within EA I've seen people apply this approach to Jacy Reese, but because the details of what he was apologizing for are vague he's only mostly kicked out and people aren't sure how to view him.

The second case is Michael Vassar, in the rationality community (with some overlap into EA). He's been mostly expelled, but not as clearly as Tsipursky/Williams/Dill. His case had (1) and (2) but not entirely:

Some alleged misbehavior was clearly bad (sexual assault) but some was strange and hard to evaluate (inducing psychosis).
The sexual assault allegation was public, but it was in a hard-to-follow Twitter thread, included accusations against other people, and mixed accusations of many levels of severity (assault, distasteful, being a bad partner). I don't fault the accuser for any of this, but as a "here's a link that explains the problems" resource it didn't work as well as the three more successful cases above.
The psychosis allegations were even harder to link, scattered across multiple threads with people changing their minds.

Vassar was widely banned (REACH, SSC), but there were still holes. For example, he was invited to speak at an online SSC meetup. With the publication of the Bloomberg article on abuse in the rationalist community, however, which contained additional allegations and also provided something clear to link to I think we now have (1) and (2) and the expulsion will stick.

Disclosure: while I'm on the BIDA Safety Team I'm speaking only for myself. My wife is on the Community Health Team at CEA, but I haven't run this post by her and don't know her views.