Not really, no. It has been mentioned before though:
How to prevent users from deleting from trash?
If this function was available, and enabled, then the user would be aware of it. ie, a message would likely appear if they tried to empty the trash informing them that this was not possible. Given that if you want to prevent the user from removing incriminating messages, it would seem better to hide the process from the user.
Zimbra does support interception for this type of thing, that may be worth looking into.