Using PHP to auto add botnet to IIS ban list (dont need help with PHP!)

Ok so I have info on how to utilize PHP to execute a command line with arguments.�  That isnt much of the problem here.

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What is happening is I have a botnet descending upon my IIS server trying to bust it on a particular script.�  I have removed all of the copies of this script, they eventually managed to break PHP on a previous server (IIS5) and drop a phishing file.� 

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Problem is, the botnet is very unsophisticated.�  They have been receiving 404 errors on the file for three months now, and I am still getting all sorts of traffic as the botnet tries to overload the script.�  I am looking to reduce the amount of processor cycles even further caused by this botnet, to give them a 403 access forbidden error instead of a 4 ...