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There is
Bug 25691 - Family mailbox: Disable empty trash and delete from trash
Certainly file another RFE for disable the delete button in trash if you wish.
You may be interested in
Bug 30582 - RFE: Recover Deleted Items ability for users (tombstone area or abiltiy to recover items not yet purged)
You could change the lifetime/purge thread but it won't help those who've emptied trash manually :
su - zimbra
zmprov mc COSname zimbraMailTrashLifetime 0
zmprov ms `zmhostname` zimbraMailPurgeSleepInterval 0
This value specifies the amount of time that the purge thread sleeps between subsequent purges. If the interval is set to 0, purging is turned off.
Mailbox Purge - Zimbra :: Wiki