To leave a Topic:
Click on [V] - [Leave] icon on the upper right of the chat window.
Press the [☰] icon on the upper right side of the chat room then click on [Leave].
Topic will disappear from the Topic list once you leave the Topic, and you won't receive any push notifications from the Topic you left.
You can re-join the Topic which you left if it is a Public Topic. For Private Topics, one of the Topic members must re-invite you.