Identifying comment area as a place to comment, and only comment

I run a site where users can submit code to algorithmic programming problems, which is then evaluated for correctness. The site has a commenting system, and the comment area looks something like this:

The problem is, users new to the site occasionally (frequently enough to be an issue) post their solutions into the comments, thinking them a way to submit their code. This is not ideal, since it spoils the problem for other users.

I first thought it might be a language barrier for non-English speaking users, but found that most of the code postings were done by users registered from English-speaking countries.

How can I make it more clear that the comment area is for commenting on problems, and not for solving problems? I was hoping the icons and headers (not to mention lack of syntax highlighting in comments) would be a deterrent, but it seems like it hasn't been sufficient.

I considered that perhaps the solution submission button is too hard to find, and users are resorting to inputting the only form they see — but the solution submission button is featured prominently on the page, in a box that scrolls with the page.

Since users typically spend a few minutes reading the problem before attempting a solution, there is plenty of time for them to recognize the big blue "Submit solution" button.

How can I help users submit, or at least deter them from doing so in the comments?