excluding a host from rule

["Chang, Andre" <achang () southernwine com>]

Can you exclude specific hosts from triggering the alert in a rule?  But
still get alerted by that rule if any other hosts try the same action.
Example you have a port scan on your network and you do not want to get
alerted by that host doing the scan but you do want to get alerted by anyone
else performing a port scan.