How do i block specific users from public forms

I have developed a system for the management of mountain bike trails. The system includes a public form whereby trail users can report trail issues or incidents. This is useful as it goes straight to our trail crew who can respond directly. We have a couple of people who like to abuse this system to submit abusive messages. They use made-up email addresses each time. Is it possible to block users while still keeping it a public form with no login?