Buck Inspire said:
Thanks Jaymus and Jason!
Hidden code in RSS? Sounds complicated, will visit to see what you mean. 
It sounds worse than it was. I use the Better Feed plugin, so I just added 'Contest code: "See, I really entered the contest!"<br>' to my feed footer. To prove you entered by subscribing you put the contest code in the thread comments.
Then, I went to the theme editor and added the following at the end of Theme Functions (right before the final ?>)
function contest_comment_filter($comment_text) {
return str_replace('See, I really entered the contest!', '[code hidden]', $comment_text);
}
add_filter('get_comment_text','contest_comment_filter');
That replaces "See, I really entered the contest!" with "[code hidden]" in any comment posted anywhere on my site.
Yes, it's still possible to cheat and get the code without subscribing, but I decided I wasn't going to worry about cheaters. Too much stress for no benefit. Most people are basically honest.