You can prevent the form from submitting with
$("#prospects_form").submit(function(e) {
e.preventDefault();
});
Of course, in the function, you can check for empty fields, and if anything doesn’t look right, e.preventDefault()
will stop the submit.
Without jQuery:
var form = document.getElementById("myForm");
function handleForm(event) { event.preventDefault(); }
form.addEventListener('submit', handleForm);