Each leader has a variable called iRazeCityProb. It's set to 0, 5, 10, 20 or 30 for all FfH 2 leaders (compared to BtS where most leaders have that little but some have it at 50 or 75).
If you set it to 1000 the city should be razed no matter what unless the specific leader has a trait that gives almost no upkeep at all. Or you can just increase the number some to make it more likely and not a guarantee.