[Plugin: Contact Form 7] Hacker try to abuse cf7

I notice Hacker attempts to abuse my cf7 script. It looks like a bot scans for the Hash #wpcf7-xxxxx-xxxx-o1, #wpcf7-xxxxx-xxxx-o2 and so on. Everyday i get a dozen of fake mails. Two questions, what can hackers do with the cf7 script in worst case? How to prevent that ?

http://wordpress.org/extend/plugins/contact-form-7/