VBScript form validation, user hits enter bypassing _OnSubmit event

I'm using the following code to prevent a user from entering a value in a text box higher than that stored in the session.  When the user hits the submit button, the script works properly, but if they hit ENTER while inside that text box the form submits bypassing any form validation...

First, here's the code:

<SCRIPT LANGUAGE = "VBSCRIPT">
Dim frm1
Dim frm2
dim frm3

sub window_onLoad
    set frm1 = document.form
    set frm2 = document.form1
    set frm3 = document.form2
end sub


sub jbno_OnChange
    frm1.submit
End sub
   
sub jaseq_OnChange
    If frm2.jaseq.value = "0" Then
        Alert "You MUST select an operation number!"
    Else
        frm2.submit
    End If
End sub

sub Submitbutton_OnClick
    jaseq = frm3.hours.value
    If frm2.jaseq.value = "0" Then
        Alert "You MUST select an operation number!"
        frm2.jaseq.focus
        Exit Sub
    ElseIf frm3.hours.value = "" or isnumeric(frm3.hours.value) = False Then
   
        Alert "You MUST enter a valid amount of time worked!"

    ElseIf frm3.hours.value > <%=session("total")%> Then
   
        Alert "You cannot enter more then the <%=session("total")%> hours left on the time card!"


            Else
        frm3.submit
    End If
End sub


Sub frm3_OnSubmit
jaseq = frm3.hours.value
    If frm2.jaseq.value = "0" Then
        Alert "You MUST select an operation number!"
        frm2.jaseq.focus
        Exit Sub
    ElseIf frm3.hours.value = "" or isnumeric(frm3.hours.value) = False Then
   
        Alert "You MUST enter a valid amount of time worked!"
    ElseIf frm3.hours.value > <%=session("total")%> Then
   
        Alert "You cannot enter more then the <%=session("total")%> hours left on the time card!"
        frm3_OnSubmit = False

        Else
        frm3.submit
    End If
End Sub
     
</SCRIPT>


Next, if I copied the contents of frm3_OnSubmit to hours_OnChange, it WORKS, generating the alert, but after they hit OK, it still submits the form.  HOW do I prevent the form from submitting?  I've tried all sorts of things like

frm3_onSubmit = False
hours_OnChange = False
Exit Sub

Etc etc.  Basically I get errors using all three methods.  However, the only time the frm3_OnSubmit work is when it is inside the frm3_OnSubmit sub.

Any ideas?

Thanks!

-Jason