How can I keep html code out of my forms?

I’ve a mail form on my website and I’m wondering how I can validate what is put into the message part of my form so that I can keep html code out of it.
Can you help me with some code to strip out html code from my form?

use javascript to exclude special characters like < and >. Check out the source. Ignore the message the guy wrote, the javascript you see should work. Just tweak it to your needs.

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