I understand your frustration especially since they keep changing the number so you can't block the number and be done.
This forum post explains why we can not block numbers on a DSL line : viewtopic.php?f=14&t=7089
One of the main information here would be on that post " I checked with voice ops team, and the issue is that the voice switching behind the copper-based Fusion network does not have per-line block-listing capabilities. It’s a technological limitation.
Our newer VoIP-based phone services on fiber and IP Broadband do have this capability, and each customer can manage their own personal block list. "
I checked the account and one way we could stop this is by upgrading you to our IPBB service. This would put you on the VoIP phone line in which you are able to block those numbers yourself in membertools in commportal.