Hey OP!
It's basically impossible to completely stop a determined spammer - there was a functionality to mute new user accounts in the old chat version that would have helped, but alas, it no longer exists. So your best bet is to keep ignoring the accounts, and if everyone does this and stops giving the spammer attention (feeding the troll), then he'll give up (the troll will starve).
In the meantime, if you refresh the chat after ignoring the guest account, all their messages will disappear from your screen. Hopefully this tip will help you to maintain conversations and keep up with the chat.
I'll tag the room's admin to get their input.
@Amaisey
Have a nice day
@GothandRock and try to enjoy your chat!