If it's a private room you can simply throw the person out but since you are using the word guest I'm guessing it's a public room or a room with Guest Access enabled. In that case there's no way of muting someone in HipChat.
There's a feature request in Atlassian's public Issue Tracker, you can vote and watch: HCPUB-3260.
All good things come to an end - thanks to all our customers and partners who have been along the Hipchat and Stride journey with us. As of Feb 15th 2019, Hipchat Cloud and Stride have reached ...
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