Triggering the website to send Spam

[Littleguru <littleguru sympatico ca>]

ello

I know that I can block the source of spam attack through
firewall , but what can I do about the sites that have been
triggered to send spam .the site is innocent but spammers are
using it for sending spam , so I dont want to suspend that site.
Do you know how to accomplish this, and how they trigger
another site to send spam ? and how we can prevent this ?
any link , help or hint will be greatly appreciated.