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/arch...-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.