First thing you need to do is disable the script that is being comprimised. I have dealt with this issue and you do not want to be blacklisted;)
The problem is that they inject line feeds and the code for the up and left arrow key to insert for example a bcc. Take a look here, to solve the line feed. http://www.gerd-riesselmann.net/archives/2005/09/sending-spam-through-contact-forms
The up arrow and stuff is a little bit harder to fix. I can't give out the code unfortunatly cause I did not write it.... But with some Googleling you could find something.
I hope you get your forms safe again.