You'll want to look into corlists. You can define the corlist on each ephone.
I think for this case you would have to define a corlist enabling access for every *other* phone, because of the way corlists works.
If you're looking for a simple way to do this, I'm not sure if it can be done.
The way I'm thinking you can get this done is to put an incoming corlist with privileges for every other phone, and for the particular number you want to block create an outgoing dialpeer with the corlist configuration.
Here's the documentation on corlists:
http://www.cisco.com/en/US/tech/tk652/tk90/technologies_configuration_example09186a008019d649.shtml
Here's an example that allows one ephone to call out a certain number:
dial-peer voice 123456 pots
corlist outgoing Group1-Outbound
destination-pattern 123456
port 0/0/3
!
dial-peer cor list Group1-Outbound
member Group1-Outbound
!
!
ephone-dn 1 dual-line
number 200
corlist incoming Group1-Outbound
!
