That is correct: SLBL only works on SMTP addresses.
If I understand correctly, the user wants to prune thier Block List by removing addresses that are not actively sending spam. This is not easy, but it is something that the user can do themselves:
The user can log directly into the Spam Quarantine to see what mail has been stopped on thier behalf recently. They would then access their SLBL through the Spam Quarntine to see what addresses they have configured for Blocking. For each address listed, they would Search their Spam Quarntine for emails from that address. They would then review the content of those emails to determine if they stilll want to block the specific address or not.
I hope this helps!
- Jackie