If you have Symposium products, you can write a script to block this call, but it means running all calls through the script for this extension. If the caller changes phones, you will have to rewrite the script to monitor for that number.
It is unusual that the telco cannot block the number, usually for a fee they will do that, but it is a monthly fee, not typically a one time cost. Then if they change numbers to call in you have to block that number too at an added cost.
Without threatening, and possibly having to follow though with legal action, you may not have a good solution here, unless you are willing to spend the money.