There is a way of doing this in the call-ui.
Granted it is not automatic but may save you writing a script.

Simply press the "envelope" button that appears on the incoming call window.
This opens a text message window for that contact.
Type or paste your "I am busy, go away" message.
Then press send, the call is automagically rejected, logged as a missed call so you have the number and the message is sent.