Yup, it's gotta be server side or it's just no worth implementing.

Since you're using Frontpage, I'd assume your server will run .asp scripts.

There are tons of free and easy to implement "CAPTCHA" scripts available. Any one of the following look like they'll meet your immediate needs. I've not used them myself so can't vouch for them, I'm simply providing a resource.

Here's one:
Captcha ASP script - prevents automated form sumbissions

Here's another:
ASP Security Image Generator (CAPTCHA)

And Another with CSS and JavaScript:
A CAPTCHA Solution For Classic ASP

Good Luck.

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