Assuming your Provider is defined on SP1.
Voice Services -> SP1 Service
Add to the beginning of X_InboundCallRoute:
{(1?8005551212):},
Change 8005551212 to the callerid of the person you want to block
The 1? will allow for 10 or 11 digit callerid's
If the curent X_InboundCallRoute route is just ph code it like this:
{(1?8005551212):},{ph}
If you want to block multiple numbers separate them with a vertical bar
{(1?8005551212|1?8005551313):},{ph}
You need to use OBi Expert to make this change.
Edit:
The provider will think the phone is ringing, so it will have to ring the set number of times before going to VM.