If they say no, I can name and shame them, and I think that's probably the most efficient dissuasion. In my experience, all public servants hate being dragged in front of a committee, so naming and shaming them and threatening that they'll be required to testify in front of a committee is sufficient in most cases to allow them to provide me with the information, if they can.
On October 2nd, 2018. See this statement in context.