The post that was quoted here has been removed
I'm not a car engineer but firstly why would someone do that? If they wanted to do damage to your car they would key the paintwork. Seems a lot of effort for not much damage. A small stone in the oil however could be an issue. It would get caught up in the overhead cam system. If this has happened it may require having the rocker cover removed (or whatever the top cover of the engine is called) and checking. What would be worse than a small stone would be sand, which would get down through the value system and bugger up the cylinders. Got some enemies??!