Perhaps you should look at scripted alerts... here
There are couple of other question you should also look at for hints ...
http://splunk-base.splunk.com/answers/3019/scripted-alert-question
AND
http://splunk-base.splunk.com/answers/40843/alerting-send-ipuser-to-script-as-a-parameter
But generally you should also look to support/forums for your firewall vendor for the actual script sections that will be needed to add firewall rules to the access list.
Hope this helps,
MHibbin
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