The archive format is not intended to be end-user editable. Plus, if you were to edit it, serv-u might not know to load the changed values into memory.
You may be able to investigate the integration dll (look at you install directory). You might be able to write a handler that forbids connections based on IP (SUCNConnect might be able to do this). It will be challenging, as writing your own dll is not easy.
I checked the DB schema, and I think you might be right- the blocked IPs are not stored there. It's documented here, if you wish to verify:
