A spambot attack blocked my forms

Dear Friends,

a spambot attacked my form: https://www.circolodifilosofia.com/prenota

Now, it don’t work anymore: If I compile it and I confirm, a generic error message appears. How could I unlock my form?

Thank you,

Francesco Baroni (Circolo di Filosofia)

The page I need help with: [log in to see the link]