Hi, Stu.
I think that if you can check your log files for and IP's of visitors that correspond to the 'submitions' you are getting, it may be a start, but pretty much it is easy to spoof all that info or block it completely.
I can only suggest that we put a special tracking log on that page, and see if it comes up with anything relevant.
The best bet would be to install a validation script that prevents duplicate submitions. I am sure we could find one easily, if you are interested.
I also found this link to an anti-spam resource page, some of the links are dead, but there is some very helpful pointers and links here as well:
http://www.rahul.net/falk/index.html#howtos
Let me know how it goes :o)