you just need to set a false return status for that method.
i've only done this with a single crap vbscript function though....
Code:
If (pform.BranchNumber.value = "" or (Len(pform.BranchNumber.value) > 6 )) Then
MsgBox "Branch Number field must contain a value, e.g. 6380", 0, "Validation Error"
pform.BranchNumber.focus()
pipcode_form_onsubmit = False
Exit Function
End If
where the name of my validation function is pipcode_form_onsubmit