Disable Mail Delivery

I have a private mail server at home, but due to the huge amount of SPAM 
being received lately I have decided to shut down the mail server. I 
have Uw-imap and sendmail running on the server. This is a multi-purpose 
server, so I can't just delete the users, and because some accounts are 
still receiving important mail I can't just turn off the imap server. 
What I want to do is stop all email delivery to accounts that I know are 
not receiving anything important, and then at some point down the road, 
when I know it's safe I'll shutdown imap.

So my question is, how do I prevent Uw-imap from receiving email for a 
specific account, but not all accounts, without deleting that account?

Thanks,
Rick
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