Matrix project lead here; We have an extensive moderation guide up at https://matrix.org/docs/guides/moderation which explains the options available on Riot/Matrix. We’re doing a lot of work in this space currently (the guide had a fairly major update yesterday); this is a massive area of focus for us.
Edit: to answer the specific questions:
- You can flag a comment by selecting ‘report’ (although this is only present currently on Riot/iOS & Riot/Android - we’re just adding it into Riot/Web.). It reports the message ID into the server DB; the server admin can then decide what to do with it.
- For moderation in general, see the guide.
- Community moderation is supported
- Content can be deleted for everyone using the ‘Remove’ button, which now also supports bulk removals
- You can make rooms invite only, or mute-by-default to mitigate flooding attacks. There are also much more sophisticated plugins and bots and tools too as per the mod guide (e.g. nsfw-filtering bots)
- You can deactivate malicious accounts as per the guide.