Someone tapped into my wireless network illegally?

My broadband usage for the last month shows unauthorised usage of my broadband wireless conection. Consequently my service provider's bill soared into the high skies!! How can I know who used it illegally? How can I prevent it from happening in the future? Is it possible to track any illegal or new user while... show more My broadband usage for the last month shows unauthorised usage of my broadband wireless conection. Consequently my service provider's bill soared into the high skies!! How can I know who used it illegally? How can I prevent it from happening in the future? Is it possible to track any illegal or new user while in the act or even after? Any good software for this?

By the way, it was a secured network with a password, still it happened. Is remote logging possible, if someone knows my account ID and password?

Thanks for your help.
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