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").
Update 2013-09-05.
I have since played two more AI box experiments after this one, winning both.
Update 2013-12-30:
I have lost two more AI box experiments, and won two more. Current Record is 3 Wins, 3 Losses.