If I understand right you think it's only coming from your mail sender page script so you only need to add some code to the top of that page to prevent the script from executing if the referral didn't come from your own contact page.
Something like this at the top of your mail sender page might help take care of the problem:
echo 'Your Message Was NOT Sent You Bad Bad Bot. Also, For Validation Purposes This Form Requires A Valid HTTP Referrer And Referrer Headers Enabled. There Was An Invalid Referrer Or You Are Using A Browser Plugin That Disables It Please Enable It, Use Your Browser Back Button, And Resubmit Your Request.';
I wouldn't use that just on it's own but that seems to be all you are asking. I would also parse the user input to cleanse it but I think you are saying you already have that in place.