Having allowed external content for a message, how do I block it again for that same message?

As per the title. I have allowed external content for one message (it is disabled by default). Finding the option to do that was easy, because there is a specific message with a drop-down menu to configure access to external content for that particular message.

However, now that I have allowed external content, I do not wish to keep it allowed (no need for external parties to know when and where I open and re-open and re-re-open this e-mail). The drop-down menu to manage the privacy options for this particular message has disappeared. That is inconvenient, to say the least, because I can't see any other way to get to those options. What am I missing?