Prevent a message from be deleted but not from be moved

Is it possible to prevent a message from be deleted but not from be moved to another mailbox via ACL?

I need this because I'm using antispam plugin with the mailtrain backend. It calls a script every time a message is moved to/from a monitored spam mailbox. But, if the user deletes a message from the spam folder, the script is executed with the option to train the antispam to consider that excluded message as a non spam.

So, what I need is to prevent the user from delete a message from the spam mailbox, but I want to permit the user to move a menssage from the spam mailbox to another mailbox.

-- Att. Bruno Galindro da Costa