disabling bulk mail

hello all,
i have a problem that requires a quick assisst. i have this program that
users run on my network that allows them send multiple mails at a time
thereby slowing down my network. i think the program they use is called
mail extractor or something. is there a way i can block this using isa? i
would really like to prevent them from sending such mails as they choke up
my bandwidth and my isp too has complained.


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