To remove the conversation from the active conversations list, you can easily do so by clicking on the conversation options icon, and clicking on Archive conversation.

Archiving the conversation only moves this to the archived conversations list and will be automatically unarchived when you receive a new message. 

Alternatively, you can also unarchive the conversation by accessing the archived conversations, clicking on the archive conversation in the conversation list and then clicking on Unarchive conversation under the conversation options menu.

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