My question is buildings can and do have wards placed around them to stop spirits from just wandering in. However for buildings without wards do spirits need to overcome the Object Threshold of the material the building is made of? After reading WAR! with its mods to increase Object Threshold I am wondering if I have been letting spirits get away with things in my game.

Nope, there are no thresholds. A spirit or projecting mage can zip right through walls and other solid objects. The Earth, being a Living Thing, does present a barrier that must be pushed through tediously.

Now, if the spirit wanted to cast a spell on an object, that would be a different story.

Interesting. Then the summoning mage simply orders his fire spirit to go inside the tank and manifest. No more crew, no more threat. The mod does help against indirect and direct spells against the tank though.

Yep. I like an Air spirit for that purpose, both for its non-lethal toxic Engulf, and for its ridiculously high speed (can catch any ground vehicle).

Also an Air spirit does not damage the vehicule itself, which can be nice if you want to be able to take control of it once the crew is disabled.

Absolutely, and Air is available from both core traditions.

My previous mage PC liked the non-lethal factor (he never killed anyone).

So spirits are still going to be the weapon of choice to take out manned vehicles.

Logically, this tactic would be common enough that 2070 tanks would be designed to minimize this risk, either by using drones in lieu of manned vehicles, or by making tanks one-man, rigger-controled vehicles with the rigger in a form-fitting cocoon and no unoccupied open space for the spirit to materialize in.

You could have a space ready for the spirit to materialize in: its a thick metal lined box with really large batteries attached, with gasoline on the floor and a couple of grenades.

Wouldn't also make sense to have the space where (meta)humans would be operating the tank from warded as well?