I am the webmaster for my girlfriend's page where she advertises her services as an online tutor. The webpage has a simple PHP post form, using which the visitors can leave their e-mail addresses to be contacted about possible lessons. Once an e-mail address is received, it is validated and then e-mailed to me. Usually there's a message once every 3-4 days.
Since yesterday I have received about 50 e-mail messages with different e-mail addresses, about once every half hour or so. I checked the IP addresses of the senders, and they are from proxy servers all over the world, so I can't just filter them out based on IP. The e-mail addresses themselves are linked to spammers (according to Google). I am assuming it is a bot and not a real person typing these e-mail addresses into the form.
Can anyone suggest how I could stop this deluge of spam? I guess I could try to figure out how to use a captcha, but, before I do it, maybe there is an easier way? Also, could anyone perhaps explain their motivation? How could they possibly expect to benefit from this?