My tactic so far has been:
1. Give Bob a massively overpowered car so he can get in the lead fairly early.
2. Ignore Bob for the entire race, until
3. On the last lap, check to make sure he's got enough time between him and the car behind. If not, tell him to hurry up and he'll pull his finger out enough to keep them at bay and not gain ground on him.
Of course, if your car is powerful enough he'll still gain ground on them even when fully chilling. I find it helps if you use the live timing screen (second button on the left of the B-Spec screen) and watch the Gap part, if that starts to fall then you need to tell him to hurry up.
It does suck that his AI is so terminally stupid that you have to give him the best car you can enter into the race, but that's just how it is (unless my driver is particularly bad).
Edit: in response to your first post, one reason for your driver placing lower than before could simply be his opposition; In the Ferrari race (on A-Spec, though) I entered a fairly modified 458 and came 3rd, because there were two Enzos in 1st and 3rd place. I tried again and came first because there was only one Enzo and it started behind me. B-Spec strikes me as being even more susceptible to this, because if you can get in the lead early on you're pretty much set as long as you keep him calm but not too calm, but if there's a faster car in front it'll be harder to keep him cool and even harder for him to pass it.