If a viewer of your live stream is disrupting your stream by spamming, or posting inappropriate comments, you have the ability to put them in a "timeout" for 5 minutes. From the StreamYard studio, click the 3 dots on the comment you want to put in timeout, then click Put user in timeout. They will be notified about the 5 minute timeout, and this can be extended if needed. Once the timeout period is over they will be able to comment again. This feature is only available for comments made on YouTube.
