[Exim] system-filter delivery help

Using Exim 4.3. I have a system-filter as follows:

# Exim filter
#
logfile /var/log/spam/spam.log

#########################################################################
# Quarantine SPAM mail
if "${if def:h_X-my-secret-flag {def}{undef}}" is "def"
or
$h_X-Spam-Status: contains "hits=5.0"
then
logwrite "\n==========================================================\nFrom: $h_From:\nSubject: $h_Subject:\nReceived: $h_Received:"
deliver spambox@???
finish
endif

Anything marked as spam gets delivered to the spambox address fine. The problem is that if spambox@??? is unavailable, exim just passes it to the original recipient - which is bad in this case (I don't want users receiving the spams). I know that if I use "seen finish" instead of finish, the mail won't get passed along - Problem with that is a bounce gets sent to the spam sender - which is bad. Is there a way to freeze the message if it fails the filter to have it try again? or at least stop the bounce?