Can I code to prevent field input?

[vics]

Gang;

As we get a lage amount of spammers per week of your typical varietal (see attached), and no, we're not going to upgrade our registration system just yet, can I prevent via field code, the numeric string '123456' in our 'Real Name' field?

This would reject the registration attempt.

If so, where is this done and what might it be?
I'd apprecaite the exact code string...

Any help appreciated - PM me if you can do it as a 'pay gig' as well.

Thanks