I would advise against using any method where the answer is always the same. A hacker only has to visit the form once to use it remotely from then on.
See this:
http://rtfm.modx.com/display/ADDON/FormIt.Hooks.math for another way to go.
Thanks for your kind support, it worked for me. And yes i implemented your suggestion of changing captcha. Thankyou so much.
I'm glad it worked for you. Thanks for reporting back