Know that we take this kind of behavior seriously, however, at this time we cannot take measures against guest accounts. I am relatively new to the staff so I am unsure if I am right in saying that all official servers have guest chatting turned off. If you need assistance with a case on a privately owned sever we do recommend that you bring it to the attention of the server owner. If this is a server that you frequent or is the server of a specific community member/fandom reporting the issue to them may encourage them to seek out plugins and management aids.
Here, on this topic, Titch introduced a plugin called “GuestControl” that may help in these situations. GuestControl - Control what you want to allow guests to do!
I hope this helps.