If you use captcha you could rely on a javascript solution, instead of using a Server Side language. But, relying on client-side solutions to filter data and prevent spam is never a good idea.

If you are using mail forms on your website the chances are high that you are already using a Server Side language like PHP, ASP, etc.

The key to good form processing is to never, ever, ever, under any circumstances, trust user data input. Filter the data for acceptable content and to prevent the form from being hijacked to send spam to other people. If you follow some sound security priciples the odds are you will likely be ok. CAPTCHA is only one tool and it can be circumvented using a variety of methods.

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I use Country IP Blocks as added security for my networks and servers.