Prevent spam through contact form

Hello friends,
Its a general thing that we put up contact forms on website for visitors to send their feedback/enquiries. I would like your views on a related issue.

With regards to security and the script used to send the email, the spammers may use it to send spam from. Some scripts used behind web based forms have been known to allow hackers/spammers to send mail from it. I do not want this to be able to happen, this is my concern regarding security.
How can this be avoided?

Looking ahead to your views.

Thanks