You can close a conversation simply by clicking on the "Close" button. You can find the "Close" button by either hovering over the avatar in the conversation list or in the conversation header, when viewing the conversation.
In order to reopen a closed conversation, you just need to click on the same button which is now the "Reopen" button.
Once a conversation is opened or closed it will be moved to the relevant tab, either "Open" or "Closed".
💡TIP: It is good practice to close a conversation that does not require any action, follow up or reply. Conversations will be automatically re-opened when a reply is received from any of the participants.