Damages to driver.

Hello everybody, I am trying to make a A7V tank in a Minecraft mod and I wanted to add a procedure that when the entity is not being ridden, then it doesn't do any damages, but if it is being ridden, it will inflict damages to everything touching it (like if you were crushing them under the vehicle) but not to the driver.
I have problem specifying that the driver should not receive any damages.

Can anybody help me ?