It's not possible in Asterisk, to reliably terminate call before it is answered. I can create external script for you which will connect to your Asterisk (or to your VoIP provider) via SIP. Since it would work on low level i would be able to terminate call as soon as i receive "Ringing" event.
However, keep in mind, that some of VoIP providers generate fake ringing tone (event) for their customers. With such providers you could have a "ringing" event. So, it's not 100% reliable - without real answer.