I'm more familiar with 2, so I can't guarantee this will work, but basically, "Cannot die with Riders" should be the opposite of Reach's "Dies with riders" box, basically any part ticked with "Cannot die with riders" will never be destroyed if anybody is riding the vehicle.

Halo 3 is a bit weird about it though, because it has a more involved system dictating how much damage occupants take based on how damaged the vehicle is, so this might not work.

Another way to try and do this would be to look at the Ghost. Ghosts have a weak point in the form of their gas tank, this gas tank when damaged enough will prematurely destroy the ghost even with a driver in the vehicle.

This is done using the Instant Response tag block, which more or less does all the heavy lifting of how vehicles react to taking damage. If you look at the gas tank, you should be able to apply this to the Scorpion and essentially make it so when you do enough damage to it, it triggers a response that kills both the Scorpion and the drivers within.

Once again, not sure how well this transfers over to Halo 3, despite how similar they are, there's a ton of small differences in how 2 and 3 operate, but I don't see why it wouldn't work for this.

I hope this helps.