I've built a plugin to provide an "enemy questions" feature. In the question bank, you can specify that if question A is on any quiz, question B should not also be added to that quiz. This works both for the random question type and manual selection of questions.
Right now, it's just working - there are no unit tests, no README file or other documentation. I'm letting people know about it right now anyway because I won't have time to clean it up and write units tests very soon, as I have to rush off to another project (PDF quizzes and Scantrons). I figured if any other devs want to useit, and maybe even help write the units tests, I'd make it available now.
https://github.com/MorrisR2/moodle_local_enemyquestions
Actually right at the moment we're also missing backup and restore functionality. I plan to have that added very soon, though I may need to ask Tim's help working with the question bank backup, in which case it's pending his schedule.
It does include a very minor patch to mod/quiz/locallib.php
If you think you would find this helpful and would like to see further development, let me know in this thread, please.