Is it possible to add the validation code for new users on that particular version?
If so, how?

What kind of validation code? That version does validate email addresses.

I think he wants a captcha for v6 of UBB.T... Which, I don't believe a captcha was made as a modification for v6 and the v7 feature would be very difficult to backport...

hmmm.. for anyone still running 6.1.0.3, CAPTCHA is the least of their security worries I'd at the very least upgrade to 6.5.5.

If there was a captcha Mod for classic I would have hacked it in the first day it was posted. I remember at UBBDev there was talk about one once but nobody ever posted some code.

Take a look here. It was made for UBBthreads version 6.0.1. Use it on your own peril.

I've been getting a lot of spam registrations recently, so have just installed that update. So far so good (though it has only been 3 minutes...).

On 6.5.5 I had to add

$userdigit = get_input("userdigit","post"); // escape in script

... in the "Get The Input" bit, otherwise it would return a "all the fields are not filled in" error.