Hi!
That's actually what I did yesterday and Andrew (one of the FreePBX devs) just applied it today...
(Thank you Andrew!!!)
The "FreePBX way" 
to do this with FreePBX 13 (if you have core 13.0.5 or later) is
"Terminate call, Play no service message"
What it does is play the SIT tone, play the "no service" message and hang up...
Have a nice day!
Nick