I was checking my logs and realized that all the comment spam I was receiving was done using http GET. Thus a quick temporary fix is to disable GET from comment postings as MT’s forms all use POST.

Just insert this little snippet at line 50 of your MT-BIN/lib/MT/App/Comment.pm file, note you can put it really more or less wherever you want in the validation part of the post method. I placed it there as thats before any real heavy work is done:

if ($q->request_method() ne "POST") {
        return $app->handle_error($app->translate("You are not allowed to post comments."));
}

Remember this wont stop spam for ever, it’s more like a temporary bandaid.