It is not possible to do this with CCM, but as already stated you can do it with your H.323 Gateway, try the following:
Use Translation rules:
config t
voice translation-rule 1
rule 1 reject /#/
rule 2 reject /#/
*where # is the number that you wish to block.
config t
voice translation-profile BLOCK_NUMBER
translate incoming 1
Assign this translation profile to the incoming dial-peer that you are using.
Hope that helps, if it does please rate the post.
//Jorge
