All logged out visitors to your site see an anti-spam word under the comment form.  To post a comment they must enter the correct anti-spam word.

The anti-spam word is designed to confirm your commenter is human.  It protects against automated spam comment scripts.

You can’t disable the anti-spam word because it is needed for security purposes.  We’ve made the anti-spam words as student friendly as we can while ensuring they protect against automated spam comment scripts.
Comment form

If students are having trouble with the anti-spam words we recommend you add your students as users to your site so they can post their comments as logged in users.  Logged in users don’t see and aren’t required to enter the anti-spam word.