My comment was mostly inspired by (known effective) real-world examples. Note that relieving anyone who shows signs of being persuaded is a de-emphasized but vital part of this policy, as is carefully vetting people before trusting them.

Actually implementing a "N people at a time" rule can be done using locks, guards and/or cryptography (note that many such algorithms are provably secure against an adversary with unlimited computing power, "information theoretic security").