OT: Hijacking an ASP Contact Us Page

Hello,

        I know this is off topic, but my ASP group hasn't been active for
awhile, so not sure where else to ask this. I have an ASP page that is
basically a contact us form. Fill out the info hit submit and then the form
emails the appropriate person with the information. Spammers have somehow
got a hold of the page and are filling it with all kinds of "good" html for
porn sites, and Viagra and the like. How do they do this and how does one
stop this from happening???

Thanks
Steve