Poltergeists can only be attacked physically while they are between possessions. A magical weapon is required as well. Poltergeists attack by throwing tantrums, and other larger objects, although they won't usually attack to hurt unless provoked. They prefer to cause trouble for as long as possible, and dead people don't care about getting hit by stones. Remember, though, that poltergeists are invisible, making it generally harder to hit them by four.
Poltergeists can haunt locations or individuals.
The individual is allowed a saving throw vs. spells, with a penalty of half the poltergeist's hit dice. Poltergeists have no control over the possessed individual, but do get to throw things at a wider variety of people. Unless they've had the bad luck to possess a hermit, in which case they'll leave as soon as they figure it out. Since they're kind of dumb, though, it may take a while. Poltergeists are stupid.