You don't say if the people living there are owners or renters, but I'm guessing they are renters? Either way, if you don't yet have any legal interest in the property there is not much you can do. If the police are there constantly, you might be able to sue them for destroying your quiet enjoyment of your property, but I'm not sure that would get you the result you want, even if you were successful. You need to consult with an attorney and go over the facts in detail.
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If I were you, I would find another house to buy.
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