How does zombie/hijacked IP address space get de-listed by SORBS?
In most cases it doesn’t. All you can do is to notify the SORBS investigators that they should reinvestigate the details of the entry. In the few cases where it is possible, you would do so by providing proof that the netblock is legally registered to you via the regional Internet registry and that SORBS has made a mistake. This is unlikely, but possible nonetheless. SORBS does not want to make mistakes and will correct any errors as soon as notified (with evidence).