Honestly, pay them more. In every major country, if they're running orgs that have multi-hundred-billion-dollar budgets per year, and you have a few hundred of these people, you can pay them well-enough that sneaking around isn't really profitable.
It would not be unreasonable to allocate 0.1% of your budget to compensating the people who control the budget, in exchange for them abandoning their conflict-of-interest positions.