If you are not seeing the option to complete a conversation, it’s because you are not the conversation owner. Only the owner of a conversation can complete the conversation. If you are not the conversation owner, you will simply see the option to Leave the conversation.
If necessary, you can use the Members button and make yourself the conversation owner in order to complete it.
If you are in a group chat within Captivated, you cannot transfer ownership to yourself and complete the conversation. Only the person who started the group chat can complete it. If necessary, you can request that the creator of the conversation remove you from the group chat.