I don't believe that's possible. PDF files were meant to be printable at multiple sizes. You can't limit them in that way.
You should inform these third-parties that you will reject any documents that are returned in an incorrect page size.
I had a feeling that might be the case. Thanks!
You cannot force somebody to use actual size, but you can make it less likely that somebody will use their default of "fit". When you bring up the document properties (Ctrl-D or Cmd-D), and then go to the "Advanced" tab, you can set defaults for some of the settings on the print dialog. The user can still override them, but that requires an action by the user. The problem you are dealing with is that the document may come up with "Fit" already selected, and the user not paying any attention to that. By setting the default in the document, you are preventing these types of mistakes.
Once "Page Scaling" is set to "None", when you bring up the document in Acrobat or the free Reader, it will default to "Actual Size".
Thanks, Karl! This is really helpful.