Consider manipulating your layers at runtime.
Enabling/disabling collisions for certain objects with entire layers
Physics.IgnoreCollision and Physics.IgnoreLayerCollision allow you to disable collisions between two colliders, and between two layers, respectively.
If you want to exclude a specific collider from collisions, there's no built in way to do that. If you are using non-kinematic rigidbodies with non-trigger colliders, there will always be a response even if you try to filter out the responses by tag.
The way we accomplished this on our project was to create a layer named "Void". Void does not collide with anything at all.
Now you can temporarily disable a collider's collision by setting its layer to "Void", and re-enable it by restoring its old layer.
Here's the gist:
int oldLayer = -1;
voidLayer = 1 << LayerMask.NameToLayer("Void");
void DisableCollider(Collider col)
oldLayer = col.gameObject.layer;
col.gameObject.layer = voidLayer;
void EnableCollider(Collider col)
col.gameObject.layer = oldLayer;
We did this specifically because enabling and disabling colliders that were children of rigidbodies can really screw things up in some circumstances. So you can't reliably disable part of a compound rigidbody collider, but you can reliably swap around the layer of one of the compound collider parts.
Now your specific situation
To reduce the impact zombie collision has on your car, consider making your zombies kinematic rigidbody triggers, or otherwise making their mass very very small (like float.Epsilon), so the impact has zero effect on the car's momentum. On impact, you can also do this layer switch, so that subsequent collisions aren't triggered, even if the first one applies a negligible force to the car body.
You could even switch them to a layer that just doesn't collide with the car.
You could also put another collider on zombies that get activated on ran-over. This way you can fake impact forces OnTriggerEnter. You can't get as accurate with it, but you can approximate where the contact normals may have been well enough to throw the zombie somewhere fun.
You have to ask yourself:
Do I want to be able to knock the ran-over zombies around with my car, or should they just play a death animation?
Do I care if the car's velocity and angular velocity are affected in smaller ways by hitting zombies?