Here's how to remove a message or attachment from a Facebook chat without deleting the entire conversation. (See all Facebook tips.)
If you've ever been chatting on Facebook from your browser and sent a message or attachment to the wrong person, or simply wished you hadn't sent it in the first place, you can remove it without deleting your entire conversation.
You'll need to be viewing the full conversation to start with, so if you're chatting in a pop-up conversation window, click the gear wheel towards the top right of the pop-up and select 'See Full Conversation'. Otherwise, you can click the Messages link in your favourites to enter your inbox as normal and click on the conversation containing the messages you want to delete. (See also: How to block app request on Facebook.)
With the relevant conversation selected, click on the Actions button at the top right and the select 'Delete Messages'. This will place a checkbox next to each message in the conversation. Now click the box next to any messages you wish to delete and press the Delete button at the bottom of the page. (See also: How to make a Facebook account private.)
Facebook will now give you one last chance to cancel the operation if you're not sure you've selected the right messages. If you're sure, go ahead and click the 'Delete messages' button to permanently remove all selected messages.