Abuse Of A Php Contact Script, someone is the bcc to send out emails

[Karpat]

Hi -

I have had a message today from my hosting company to tell me that one of the sites on my hosting account is having it's php code abused. Apparently someone is manipulating the php code from the contact form to allow them
to add Bcc addresses.

Any ideas on what I need to do to close this loop hole?

Thanks.