This can be done within the cPanel interface. Under Mail Manager -> Default Address Maintenance, add this to your domain:
:fail: no such address here
This will result in rejects like the following:
2007-12-16 10:32:00 H=spammer.example.net (706167c405e3d8) [192.168.42.101] F=<spambot[at]example.com> rejected RCPT <me[at]mydomain.example.org>: no such address here
Royce