What I suggested is 'pseudo-code', and just part of the function that could be used for client-side validation. Without knowing things like form name/id, etc., I can't really help any more than that.
Basically, you could create a function between <script> and </script> tags which would be placed just prior to the closing </body> tag. If this is the only input that needs specialized validation, then you would make the function with just that one input in mind, so it would be a short script. Then give the cfform an onsubmit event that would check the value before submitting, and if it doesn't match what you need, "return false;" on the submit.
But this is becoming a little more complicated than I think it needs to be. I wonder if CF's built-in validation might work with modulus. I'll check around.
If anyone knows of a website that can offer ColdFusion help that isn't controlled by neurotic, pedantic jerks* (stackoverflow.com), please PM me with a link.
* The neurotic, pedantic jerks are not the owners; just the people who are in control of the "popularity contest".