[exim] Reject Specific users...

I want to be able to reject/remove specific RCPT address based on some
rules. I have the rules written and working, but I've tried implementing
this in the acl_smtp_rcpt acl and it's not working as I expected. From
within the acl, I do a DENY for addresses I wish to reject. This seems to
generate a 550 error which kills the message.

What am I missing?? Do I have to do this in the transport section?

Thanks in advance.

Dan