This sounds like a Thunderbird feature, please see: Remote Content in Messages

The link suggested explains how to establish the exception, but not how to reverse one of the options.

The three options are:

   Show remote content in this message 
   Edit remote content preferences
   Allow remote content for [this sender's email address]

The link explains how to reverse the third choice, but does not explain how to reverse the first. One I've clicked "Show remote content in this message", I found no way to reverse the choice. I want to again block the remote content for an individual message.

I'm wondering why you'd want to keep a message where you also wanted to block remote content. I'd have deleted it by now. ;-)

The "show content in this message" is a one-off; it doesn't (AFAIK) place a block on future messages from this particular sender.

No promises, but look at Tools|Options|Privacy|Mail Content→Exceptions - this where permanent blocks are implemented; it may just be listed there.

Also, if this sender has an entry in your Address Book, there is an "allow remote content" option there that might influence the display.

But I suspect what you really need to change is the entry in the msf file, and that's not something I know enough about.