>>> This is a business phone and we had 300 blank messages from this number and it filled our mailbox up. <<<

If you have a switchboard or PBX, you should be able to program it so that this number never gets through at all -- in other words, it is rejected with no indication to the caller whatsoever that any phone was ever picked up, i.e., there is no "Answer supervision."  Most VoIP phone services also provide this feature.  You might find it advertised as "selective call rejection."  (Call "blocking" usually refers to outgoing blocking, but the feature is sometimes called that, too.)  If you have this feature, you can usually drop the call or send a busy signal to the caller without your mailbox ever being presented to the caller.

If you cannot or don't want to go that far, you can always use an IVR (interactive voice response) program on all incoming calls.  It can be as simple as "Press 3 to continue," whereas pressing anything else causes the call to be rejected.  At least one VoIP company offers this as a solution to incoming junk calls and asserts that it is quite effective.  That company also allows you to use a fixed digit for the caller to press or one chosen at random, per call.  The disadvantage of this system is that it will cause answer supervision to be sent to the caller (so he'll know that the line is assigned and "live"), and, if you pay per minute to receive calls, you will be billed from the time the system picks up the call to the time it is dropped.  If the person calls many times, this might get expensive.  Decades ago, a company called "International Mobile Machines" sold a device called a "Privecall" which did essentially the same thing.  I doubt that it's still available (except as an antique), but you might be able to find it or a similar device for a regular phone.

You may want to seach the Internet for these and other solutions to your problem.