In rare cases when there's someone misbehaving in the chat, you can block this person by right-clicking on their name from the Chat or Participants tab, and selecting "Block".

Their chat messages will become invisible to all stream participants, speakers, and of course, you.

You can see all blocked viewers, and unblock them at any time from the Participants tab -- just scroll all the way down to the "Blocked" section, and right-click on their name again to see the option.