To unsubscribe from a conversation with more than one friend:

1. Open the conversation.
2. Open the Actions menu.
3. Select "Leave Conversation."

The other people in the conversation will be notified that you left and you will no longer receive messages from the conversation.

Note that you can only unsubscribe from group conversations — you can’t leave one-on-one conversations. To prevent an individual from sending you messages, you should consider unfriending or blocking them.