Has anyone seen this before?

I have a website that has a form for my clients to fill out and lately I have been recieving a form submission every day with some rather strange stuff on it. I was wondering if there was a way to trace the submitter of the form and if anyone knew of someway to restrict access to the form until the senders email was verified. I have a somewhat strong statement against porn on the form and I think the recent submissions are in retalliation. Any Help?
Here is the form submission;