well that's the default behaviour of a submit button, and I don't know any way to turn it off. You could instead use a regular button and some javascript to do the form submission
<button type="button" onclick="document.formname.submit();">Submit</button>
but then you have to consider that if any users have
JS turned off, or they don't have a
JS-enabled browser, then they won't be able to submit the info to you at all.
Can't you just handle the premature submissions by server-side validation?
hth
Phil