Not quite as thorough as Forcing someone to read a thread, which I thing has a usefulness unto it's own.
This idea would trigger an alert to everyone - the message could be to tell them to "Read thread X now".
Maybe do another read the thread now warning.
After that, where they've had sufficient warning and chances ... maybe this force users idea would be appropriate.
But maybe an alert to everyone is a better idea to start:
http://xenforo.com/community/thread...to-trigger-an-alert-when-posting-a-post.7558/