You can only block ALL calls going to voice mail, not particular number.
It is a trick involving extended absence.
To "disable" voicemail, dial *004*8080000000#
This will cause all calls will either ring certain number of times and then stop or get fast busy at the point they normally would have been diverted to the voicemail system.
Whenever you need to reset it back to defaults, dial #004#
I applied this trick few months ago and never ever got any spam voicemail in my box. Mind you, NOBODY will be able to leave vmail.
