Do you use any form of CAPTCHA such as from Google?  If not, I suggest doing so.  That tends to cut down on the Spam.

You could also edit the form code to limit the number of submissions by WAN IP however true hackers will be able to spoof their IP or use compromised computers to hit your form.

You could adjust your submission code to check to see if the same or similar values are being entered into the fields before actually passing to the database.  In my experience the robot tends to paste the same value into most of the form fields.