Griefers are a hazard of being on any kind of social network, or blog if it has comments enabled. So in Epicyon I've used a few methods to mitigate annoyances.
The easiest way for someone to do a denial of service would just be to send a gigantic post. Hundreds of megabytes or larger, and have your server clogged up trying to process it. Most often this kind of problem is mitigated by the web server configuration, but in Epicyon there's also a maximum overall message size of 20K. That includes all the json formatting.
This is one of those things which is really a fediverse standard, but isn't in the ActivityPub specification. Combining this with permanent signing keys gives a strong assurance that messages are coming from the account you think they are.
Adversarial instances can try to do blind key rotation and pretend to be someone else, but since public keys are only fetched once they're not going to succeed and messages from accounts doing that will be rejected by the signature check.
Blocklists and federation lists
Usually the most controversial aspect of the fediverse. Fights over who is blocking who are frequent. In Epicyon blocking can be global to the instance and also local to particular accounts. Global blocks override account level ones. Federation lists are the opposite, in which you are choosing to only federate with specified other instances. That can be useful if you wanted to deploy a fediverse-like system in a company or school.
A hellthread is when someone mentions you in a message containing a very large number of other mentions. In 2016 when I was running GNU Social this happened quite often. Even an upper bound of 20K is room for a lot of mentions. In Epicyon there's a configurable threshold for the maximum number of mentions. Anything above the threshold and the message will be rejected.
Similar to hellthread mitigation. An adversary can simply send you posts packed with emoji. So there's a threshold for the maximum number of emoji which a received post can contain.
This is part of the ActivityPub specification and is optional. Sometimes also called "locked account". Being able to approve the people who are following you can avoid tears later. In Epicyon if follower approval is enabled then you can just select the link to the profile of the request and see if their timeline makes sense. Have they made any recent posts which are interesting? Is their timeline full of chuds using dog whistle catchphrases? Are they being followed by or interacting with spooks or neo-nazis?
In Epicyon you can also restrict incoming DMs to only people that you follow. It's a feature blatantly copied from Twitter and is intended to mitigate the driveby griefer problem. If you're not interested in having random chuds from the interwebs send you their latest ALLCAPS hot take about why they are now convinced beyond reasonable doubt that the Earth is flat then this can save you time and disk storage.
The Reply Guy
Constantly replying to a message long after the point at which it made any sense to do so was a common griefer tactic in the past. So in Epicyon there's an upper limit on the number of replies a post can have. The reply guy won't be able to send you his 100th sealioning sermon about why you need to immediately provide him with painstakingly researched evidence for some earlier flippant remark, cited in the peer-reviewed academic journal of their choosing.
If adversaries always use common catchphrases, or if you just have zero interest in certain politicians or celebrities, then these things can be added to the word filter.
Maybe someone is not deliberately griefing you but they're just having a bad hair day. You don't want to unfollow or block because they're mostly ok. In this situation you can hit the snooze button and not hear from them again for 24 hours. Their posts won't be deleted, just made invisible for a while, and after the time is up you could go back to see what they were ranting about yesterday if you were thus inclined to do so.
Mute is something quick and easy you can do to not show the content of an individual post. So if some un-CW'd photo is annoying you then you can quickly remove it from the timeline.