One channel, one version of the truth
Name a single contact for the process and require all communication to go through them. It sounds restrictive and it is the most effective probity measure available, because it prevents the situation where one bidder has a useful conversation with a building manager and another does not. Every question and every answer then exists in one place and can be circulated. It also gives whoever holds the process a clear position when a bidder approaches someone else in the organisation.
Ask bidders not to approach decision makers directly during the process, and say so in the documents. Most will comply readily. The value is not in catching anyone out but in removing the perception of advantage, which is what an unsuccessful bidder or an internal reviewer will focus on if the outcome is ever questioned. Stating the expectation in the documents is usually enough; very few bidders test it once it has been written down.
