Nothing,
neither going deep through amavis configuration I've found how it works correctly. There is REALLY no way to say "Zimbra, please, if you tag a message as spam in subject, deliver messages into inbox instead moving them to junk!" ?
This is really quiet important becouse a lot of customers have different way to read email and not all understand what a webmail is...
EDIT:
I thought to reversing the problem: probably I can wait that Zimbra do its dirty work with spam, wait for message tagged into junk and find a way to move again messages from junk to inbox in someway ... really don't know how but if it is impossible to do front... what do you think about?