If one or more participants in an event cause feedback or echo through a mic left open, or through poor echo cancellation at their end, then you can stop that via:

1. Tell them via private chat/IM somethimg like "I am sorry, you seem to have an open mic, or when your mic is on I am hearing echo. You probably will not hear the problem yourself. So for now I will mute you and will not hear you again in this event".
2. Right click on the avatar and select "Mute".
3. The avatar will show as "Muted" in their status/group label.
4. At the end of the meeting you could unmute them in case they fix the problem for next time.