[Exim] How do I move spam around?!

Please, please, please, can someone help me with what should be a very
simple problem?

I've set up exim with spamassassin to catch spam. The scanning is
working fine, and it adds various headers to the email to indicate it's
spam rating.

The problem is that after rating the spam, it then delivers it anyway.

What I need to do is decide whether to deliver it to a user or not,
based on the spam rating. If not delivering to the user, I need to put
the mail in quarantine until the administrator has checked it to decide
if it is spam or not.

How can I achieve this? I have tried messing about with system filters,
but I don't want to freeze the message nor do I want to bounce it, nor
do I want to add it to a spool file. Ideally I want to put it into a
separate mail queue that doesn't get flushed automatically.

Please help - I've been trying to get this damn thing working for a
month and it's driving me to distraction!

Thanks

Paul.