In actuality you are not blocking the caller from calling in, you are just routing the call from that specific caller ID to whatever destination you choose. You can also route calls with no CLID, or witheld CLID in specific fashions via ICR.
From my experience route them to an AA (duplicate of the main AA with menu options edited) that allows them to be transferred to only an unwanted calls no name recorded VM box no matter what they try to transfer themselves to. Just busy seems to eventually have them call from another number, get through, and circumvent your diverting them. If they think they got through to the VM, they will just keep calling the bogus VM box unknowingly. When they get no response from their message they will simply stop calling eventually.